Optical system

ABSTRACT

Method/system locate external articles using source, detector (PSD), entrance aperture, and magnifying/reducing afocal element—expanding FOR&gt;90°, or refining precision. Between (1) source or detector and (2) aperture, at least one plural-axis-rotatable mirror addresses source/detector throughout FOR. ½- to 15-centimeter mirror enables ˜25 to ˜45 μradian beam divergence. Aperture, afocal element, and mirror(s) define source-detector path. Mirror(s) rotate in refractory- (or air/magnetic-) bearing mount; or mirror array. Auxiliary optics illuminate mirror back, monitoring return to measure (null-balance feedback) angle. To optimize imaging, auxiliary radiation propagates via splitters toward array (paralleling measurement paths), then focusing on imaging detector. Focal quality is developed as a PSF, optimized vs. angle; stored results later recover optima. Mirror drive uses magnet(s) on mirror(s). “Piston” motion yields in-phase wavefronts, so array dimensions set diffraction limit. Also: destructive reply; scaling optimizes acceleration vs. thickness; passive systems.

RELATION BACK

This document is a continuation-in-part of, and claims priority of, U.S.“regular” (nonprovisional) application Ser. No. 11/796,603 of Kane,entitled “OPTICAL SYSTEMS AND METHODS USING LARGEMICROELECTROMECHANICAL-SYSTEMS MIRRORS” and filed Apr. 28, 2007 now U.S.Pat. No. 7,732,751; and of its parent application Ser. No. 11/151,594,filed Jun. 13, 2005, now U.S. Pat. No. 7,297,934 of Kane, entitled“OPTICAL SYSTEM”, together with their precursor provisional andinternational applications.

This document also claims priority of and coowned U.S. provisionalpatent applications 60/999,159 of Hunt and Hillman et al., entitled“OPTICAL SYSTEM WITH MINIATURE SCANNING MIRROR” and filed Oct. 15, 2007,and 61/125,915 of Paul et al., entitled “ELECTROOPTICAL SENSORTECHNOLOGY . . . ” and filed Apr. 30, 2008.

All these four patent documents, and further documents incorporatedtherein, are wholly incorporated by reference into this presentdocument.

BACKGROUND

Portions of this invention are very closely related to earlier coownedpatent documents directed to optical systems and methods for imaging,and for noticing and optically following a large variety of objectsoutside an optical system. These innovations included capabilities for“steering” of radiation beams, using any of a great variety ofoptical-deflection or -switching arrangements.

Such arrangements comprised using pointable mirrors of many differenttypes, and other kinds of routing devices such as an optical-switch“fabric”, and birefringent and other nonlinear materials, all generallypositioned within an optical system. The mirrors included individualreflectors, and reflector arrays, over a broad range of sizes andtypically controllable in two axes of rotation as well as in some casespiston movement.

Some of the mirrors were microelectromechanical system is (“MEMS”) unitsor other micromechanical devices—i. e. not limited to electrical orelectronic control. Among the relatively larger mirrors (for instancethose over 5 mm across) were magnetically driven individually steeringmirrors using, for example, custom jewel bearings—or etched monoiliconin-plane torsion hinges (or “flexures”).

The present invention is not limited to teachings in those earlierdocuments. Mirror adjustments by galvanometer scanner and other steeringsystems are also applicable. Among these earlier documents are teachingsof a proprietary CatsEye™ object-warning system. Those documents teachadvanced and excellent apparatus and methods for imaging from aircraftand many other kinds of mounting arrangements, both vehicular andstationary, and in many useful practical applications encompassing,merely by way of example, commercial-airline flight-control imaging e.g. from fixed towers, astronautical rendezvous, ground-planned defensemaneuvers, and vehicle collision avoidance, as well as terrain mappingfrom space.

More specifically the above-mentioned earlier documents teach suchinnovations with greater field of regard (“FOR”) and field of view(“FOV”) than in prior approaches, and with much more nimble andsophisticated capability to notice and optically follow a large varietyof objects outside the optical system than previously possible. Even thetechnologies in those coowned documents, however, leave something to bedesired in ability to very quickly steer radiation beams whilemaintaining the beams at a fine degree of collimation and accordinglymaintaining the capability to bring the beams to a very sharp focus.

In this connection the ability to prevent degradation due to diffractionis very important. As our earlier documents show, the fundamental limitimposed by diffraction can be mitigated by use of devices (such asmirrors) that have large apertures, and this is the reason for ourprevious emphasis on relatively “large” mirrors—but in particular,mirrors up to only a centimeter across.

Likewise our earlier work has emphasized operating steering mirrors insuch a way as to yield diffraction characteristics controlled byentire-array dimensions rather than individual-mirror dimensions. Theselatter techniques do not change the fundamental relationships thatgovern the diffraction limit (i. e., larger apertures still lead tofiner collimation and focus). Rather these techniques modify thefunctioning of a mirror array to exploit those fundamental relationshipsmuch more easily—by increasing dimensions of the array rather than anindividual mirror.

Our earlier developments, however, have not fully used the availableperformance advantages of large mirrors and multimirror arrays.

Neither earlier MEMS devices nor other steering-mirror concepts providethe adequately increased aperture that is needed for best diffractioncontrol. Analogously, earlier relatively large-aperture MEMS devicescannot provide translational stability in the X, Y, and Z axes. Anothermajor inadequacy in prior-art steering deflectors has turned out to bevulnerability to vibration—particularly in high-vibration environments.Two still-further difficulties have been the relatively high power drainrequired to drive the deflectors, and relatively high mass, weight andbulk of prior gimbal systems. Especially important, in addition toaperture size, is the relatively narrow angular range (field of regard,“FOR”) of prior steering devices.

Certain patent documents have been adduced that at first sight may seemrelevant in this field. They include European patent documents and oneJapanese patent abstract of two Japanese inventors:

-   Masayoshi Esashi, with Nippon Signal in Tokyo—particularly in    European Patent Application O 686 863 A1 at PDF pages 33 and 34    (FIGS. 14 and 15); and-   Norihiro Asada, with Nihon Shingo Kabushiki Kaisha—notably in    European Patent Application EP 0 774 681 A1 at PDF pages 10, 17 and    39 (FIGS. 1, 9 and 32)—and Asada's EP 0 778 657 A1, at PDF pages 7    through 9 (FIGS. 1 through 4); and Japanese publication 08-166289 of    Jun. 25, 1996 in Patent Abstracts of Japan, Application number    06-310657 of The Nippon Signal Company Ltd.

At least preliminarily it appears that these Japanese inventors havepulled the rug out from under certain of the Draper patents. It isunclear whether any of the Draper claims survives these earlier Japaneseinventions. Four other patents of potential interest are U.S. Pat. No.3,742,238, and U.S. Pat. No. 4,658,140 (“Infrared scanner for forwardloading infrared device”), 4,470,562 (“Polaris Guidance System”) andU.S. Pat. No. 5,270,792 (“Dynamic Lateral shearing interferometer”).

Conclusion—As noted above, the present state of the art in imaging,while admirable, leaves considerable refinement to be desired.

SUMMARY OF THE DISCLOSURE

The present invention provides just such refinement. This invention hasseveral different facets or aspects that are capable of useindependently of one another; but most of them are also amenable topractice in combination together.

In preferred embodiments of a first one of its major independent facetor aspect, the invention is an optical system dynamically determiningassociated angular direction throughout a specified range of angulardirections, of an external article in a volume outside the system. Theoptical system includes a radiation source, an optical detector, anentrance aperture, and an afocal element.

The afocal element is associated with the aperture, enlarging the fieldof regard of the external article and the volume as seen by the sourceand detector. Also in the system, disposed along an optical path between(1) selectively, the source or detector and (2) the entrance aperture,is at least one mirror, rotatable about plural axes and causing thesource and detector to address varying portions of the volume outsidethe optical system. Each mirror of the at least one mirror hasdimensions in a range exceeding five millimeters.

Due to the enlarging of the field of regard together with rotation ofthe at least one mirror, and substantially without changing magnitude ofthe enlarging, the external article receives radiation from the sourceand returns the radiation to the detector throughout the specifiedrange. The aperture, afocal element, and at least one mirror togetherform a common optical path for the radiation from the source and to thedetector.

The foregoing may represent a description or definition of the firstaspect or facet of the invention in its broadest or most general form.Even as couched in these broad terms, however, it can be seen that thisfacet of the invention importantly advances the art.

In particular, by virtue of its increased aperture, relative toapertures of known prior-art optical systems (generally below fivemillimeters across), this aspect of the present invention providessignificantly finer diffraction limit—and thereby greatly refinedpointing precision and accuracy, as well as better collimation andsharper focal properties.

Although the first major aspect of the invention thus significantlyadvances the art, nevertheless to optimize enjoyment of its benefitspreferably the invention is practiced in conjunction with certainadditional features or characteristics. Preferably (but this is only apreference) the range of mirror dimensions does not exceed fifteencentimeters. We believe that the principles of our invention areapplicable to optical-system design for mirrors greater than 15 cm, butfor the present we have no commercial interest in such large systems.

One preferred additional characteristic is that the mirror dimensions bein a range exceeding one centimeter. We prefer more strongly that themirror dimensions be in a range exceeding five centimeters. We preferstill more strongly that the mirror dimensions be in a range exceedingten centimeters.

We also prefer that the at least one mirror include a two-axis mirror ina gimbal-like mount having jewel, ceramic or other refractory bearings.The term “refractory” is discussed in a “Detailed Description” sectionbelow.

For purposes of this document, and particularly in certain of theappended claims, the term “gimbal-like” shall be understood to mean thatthe indicated element (such as an inner ring, or an intermediate ring orstage of a mirror mount) is analogous in some regard to a correspondingelement of an ordinary, old-fashioned gimbal. In most such phrasing, theanalogy specifically consists of the recited element being anintermediate stage between two other stages, as in an ordinary gimbal.

In this context, the two other stages are:

-   -   an outer ring, or outer stage, that defines a first axis of        rotation about which the intermediate stage operates; and    -   an inner element or stage (here ordinarily a mirror mounted to        or integrated with a magnet) that rotates about a second axis of        rotation defined by the intermediate stage.

Thus it may be said that the inner element is to the intermediate stageas the latter is to the outer stage. In addition the two axes ofrotation are distinct and different from each other and ordinarilyorthogonal, i. e. at right angles—so that the combined effect of the tworotations is to enable scanning substantially throughout a fullthree-dimensional solid angle.

Another basic preference is that the optical system further include somemeans for monitoring mirror position to develop positional or rotationalfeedback information used in rotating the at least one mirror. In thiscase the monitoring means still further include an auxiliary opticalsystem that directs an auxiliary radiation beam to the back of themirror, and responds to the auxiliary beam after return from the back ofthe mirror, to determine rotational angle of the mirror. Yet anotherbasic preference is that the at least one mirror include a two-axismirror supported in a gimbal-like mount having air bearings or magneticbearings.

Another basic preference is that the optical detector is aposition-sensing detector (PSD) that detects radiation returned from theexternal article and tracks the article by inducing rotation of the atleast one mirror to maintain the article centered on the detector. Inthis case a subpreference is that the mirror dimensions be large enoughto sharpen the diffraction limit sufficiently for beam divergence on theorder of forty to fifty microradians or finer, at visible wavelengths.If this latter preference is observed, then still-further preferably themaximum transverse dimensions of the mirror are on the order of threecentimeters.

Alternatively to the preference for 40 to 50 μrad, stated just above, weprefer that the at least one mirror include an array of mirrors havingoverall array dimensions large enough to sharpen the diffraction limitsufficiently for beam divergence on the order of twenty-five to thirtymicroradians, at visible wavelengths. In this case the maximumtransverse dimensions of the mirror array are preferably on the order offive centimeters.

Yet another basic preference is that the “at least one mirror” include amirror array addressing the varying external portions of the volumeoutside the optical system; and further include a detector that detectsradiation from the external article and tracks the article by inducingrotation of the reflector to maintain the article centered on thedetector. According to this same basic preference the system alsoincludes some means for enabling direct measurement and optimization ofimaging quality; the enabling means include an auxiliary optical systememulating the behavior of light paths, to or from the varying externalportions, at the array; the auxiliary optical system includes a laserdirecting an auxiliary beam to a first beam-splitter that forwards aportion of the beam energy toward the mirror array, parallel to thelight paths to or from the varying external portions, and from the arrayto focus on an imaging detector; and some means for developing focalquality at the imaging detector as a point spread function, and usingthe point spread function as a figure of merit for adjustments of themirror.

In event the tracking and rotation-inducing preference is observed, thenpreferably the developing and using means include—first—some means forperturbing mirror adjustments to optimize the point spread function, forplural steering angles of the array, and storing the optimized results;and—second—some means for retrieving those mirror adjustments for eachoptimized point spread function, to reset the adjustments for anydesired steering-angle combination (i. e. without the need to repeat theoptimization).

Now in preferred embodiments of its second major independent facet oraspect, the invention is somewhat similar to the above-introduced firstindependent facet—but differs particularly in these very important ways:

-   -   its afocal element can either enlarge or reduce the field of        regard of the external article, so that this second aspect of        the invention has broader utility (as explained elsewhere in        this document, a reducing element can be employed to further        sharpen pointing precision); and    -   each mirror of the “at least one mirror” includes a mirror in a        steerable mount having jewel, ceramic or other refractory        bearings.        In this latter regard, the second aspect of the invention is        thus related to one of the above-stated preferences for the        first aspect. Further information about this second facet of our        invention may be read from the corresponding appended claim.

The foregoing may represent a description or definition of the secondaspect or facet of the invention in its broadest or most general form.Even as couched in these broad terms, however, it can be seen that thisfacet of the invention importantly advances the art.

In particular using refractory bearings can very greatly reduce frictionin the mechanism—and thereby make an enormous difference in thepotential speed of pointing adjustment for the mirror. Such a refinementessentially transports the entire optical system into an entirely newregime of capabilities, far exceeding what can be achieved with priorsystems.

It is not necessary to choose between the response-speed benefits thusprovided and the pointing precision of the first aspect of theinvention. These features are fully compatible and can be used together.

Although the second aspect of our invention thus significantly advancesthe art, nevertheless to optimize enjoyment of its benefits preferablythe invention is practiced in conjunction with certain additionalfeatures or characteristics. Preferably the includes some means forcontrolling each said at least one mirror in rotation about the pluralaxes, to detect and track such external article or articles.

The controlling means preferably include a magnet fixed to or integralwith the mirror; and some means for applying variable magnetic fields tointeract with the magnet and so magnetically develop torque that rotatesthe mirror. In addition we prefer that the controlling means alsoinclude some means for monitoring the mirror position, to developpositional or rotational feedback information used in rotating themirror.

These monitoring means in turn preferably include an auxiliary opticalthat directs an auxiliary radiation beam to the back of the mirror andresponds to the auxiliary beam after return from the back of the mirror,to determine rotational angle of the mirror. For instance in onepreferred embodiment the auxiliary optical includes a position-sensingdetector (PSD) that determines displacement of the returned beam at theback of the mirror. In an alternative preferred embodiment the auxiliaryoptical includes an interferometer that counts fringes to determineposition of the mirror directly.

Another basic preference is that the optical include some means forcontrolling each mirror (of the “at least one mirror”) in rotation aboutthe plural axes, to follow a raster pattern for imaging at leastportions of the external volume. In this case a subpreference is thatthe raster pattern be a spiral pattern. A further-nested preference isthat the spiral raster pattern reverse direction, as between outward andinward spiraling, for alternate passes through the pattern.

Still according to the second aspect of our invention, we further preferto include a beam-splitter operating to tap a wide-field-of-regard,high-resolution image out of the optical—more specifically, out of themain optical path—to a first imaging detector, at a point beforeincoming radiation in the reaches the at least one mirror. In this casea second imaging detector receives a narrow-field-of-view,high-resolution image from the at least one mirror.

This also preferably includes some means for interpreting resultingelectronic signals from both imaging detectors, to display two nestedportions of the volume on a single common visual monitor; and some meansfor controlling the mirror to position the narrow-field-of-view,high-resolution image selectively within the wide-field-of-regard,high-resolution image on the single monitor.

In preferred embodiments of its third major independent facet or aspect,our invention is a method of operating an optical system that has adual-axis MEMS steering array. The array is made up, at least in part,of individual mirrors having: (1) transverse dimensions exceeding onecentimeter, and (2) separate magnetic-field-inducing coils at oppositesides of each rotation axis, and (3) another magnet to interact withmagnetic fields induced by the coils. Having this apparatus in mind, themethod itself includes the steps of:

-   -   directing electrical currents to the separate coils of each        mirror, to produce for each mirror at least two components of        force, including:        -   a pair of variable forces directed in opposite linear            directions, applying variable torque to the mirror, and        -   an additional variable net force thrusting the mirror            outward from, or drawing the mirror inward toward, a rest            plane or backing of the array as variable piston movement;    -   passing a light beam through an afocal optic to change        magnification of the steering-array rotations; and    -   determining characteristics of the light beam when received or        transmitted.        The foregoing may represent a description or definition of the        third aspect or facet of the invention in its broadest or most        general form. Even as couched in these broad terms, however, it        can be seen that this facet of the invention importantly        advances the art.

In particular, operating the mirrors in this way enables the array to beused for diffraction control based upon the overall array dimensionrather than only the dimensions of individual mirrors. Remarkably, thediffraction-limit improvement thus obtained can be as great as an orderof magnitude.

Although the third major aspect of the invention thus significantlyadvances the art, nevertheless to optimize enjoyment of its benefitspreferably the invention is practiced in conjunction with certainadditional features or characteristics. In particular, preferably themethod further includes a preliminary step of selecting a maximumtransverse array dimension of approximately three centimeters orgreater. This choice can cause the resolution to be on the order offorty to fifty microradians, or finer.

A further preference is that the preliminary step include selecting themaximum transverse array dimension as approximately five centimeters orgreater. Yet further the preliminary step preferably include selectingthe maximum transverse array dimension as approximately ten centimetersor greater. (As mentioned elsewhere, we prefer to select a maximumtransverse array dimension not exceeding fifteen centimeters.)

Another preference is that the method exploit the potential benefitsdescribed above, by including steps of adjusting the two forcecomponents so that: (1) the several mirrors direct a light beam in adesired substantially common direction; and (20 light-beam wavefrontportions from adjacent mirrors are generally in phase, to actuallyachieve a diffraction limit conditioned by the array dimension ratherthan by the individual mirror dimensions. In this case, as explained inthe “Detailed Description” later in this document, preferably thegenerally-in-phase light-beam wavefront portions are in phase withinroughly ten to twenty percent of one wavelength.

In preferred embodiments of its fourth major independent facet oraspect, the invention is a system method of operating an optical systemthat has a laser source for transmitting a radiation beam to an externalobject, and has a dual-axis steering mirror with overall transversedimensions exceeding one centimeter, rotatable on jewel, ceramic orother refractory bearings. The method itself includes the steps of:

first, utilizing the dual-axis steering device to receive, and measurean incident angle of, an incident ray from the external object; and

second, utilizing the dual-axis steering device to direct such aradiation beam from the laser source toward the external object inresponse to the received and measured incident ray.

The foregoing may represent a description or definition of the fourthaspect or facet of the invention in its broadest or most general form.Even as couched in these broad terms, however, it can be seen that thisfacet of the invention importantly advances the art.

In particular, this facet of the invention causes the refractory-bearingadvantages of extremely high adjustment speed to be available forpurposes of directing reply beams to remote objects. As noted earlierthis feature is extremely valuable.

Although the fourth major aspect of the invention thus significantlyadvances the art, nevertheless to optimize enjoyment of its benefitspreferably the invention is practiced in conjunction with certainadditional features or characteristics. In particular, preferably thesecond utilizing step preferably includes directing such a radiationbeam to disrupt functioning, or impair structural integrity of theexternal object.

Another preference is that the first utilizing step include operatingthe mirror at a peak of acceleration as a function of mirror thickness.If this preference is observed, then in turn preferably the firstutilizing step includes preparing the mirror and magnet with a diameterof roughly one centimeter and thickness of roughly two or threemillimeters.

Alternatively if it is not desired to use a mirror of diameter onecentimeter, then preferably the first utilizing step includes the stepof preparing the mirror with diameter and thickness scaled from thatdiameter of roughly one centimeter and thickness of roughly two or threemillimeters, generally:

-   -   in inverse proportion to fourth power of mirror-and-magnet        diameter, to account for inertia; and    -   in linear proportion to mirror-and-magnet diameter, to account        for location of application of most driving force;    -   for approximate net inverse proportion to the cube of the        diameter, subject to further adjustment for increased flux in        the magnet arising from such increased thickness.

Another preference is that the first utilizing step include operatingthe mirror at a minimum of response time as a function of mirrorthickness.

In preferred embodiments of its fifth major independent facet or aspect,the invention is an optical system dynamically determining associatedangular direction throughout a specified range of angular directions, ofan external article in a volume outside the system. This optical systemincludes a radiation source, an optical detector, an entrance apertureand an afocal element, associated with the aperture. The latter elementenlarges or reduces the field of regard of the external article and thevolume as seen by the source and detector.

Also part of the system—and disposed along an optical path between (1)selectively, the source or detector and (2) the entrance aperture—is atleast one mirror rotatable about plural axes and causing the source anddetector to address varying portions of the volume outside the opticalsystem. Each mirror of the at least one mirror has dimensions in a rangeexceeding five millimeters.

When the afocal element is enlarging the field of regard, together withrotation of the at least one mirror, and substantially without changingmagnitude of the enlarging, the external article receives radiation fromthe source and returns the radiation to the detector throughout thespecified range. When the afocal element is reducing the field ofregard, pointing and steering precision is made finer by the ratio ofreduction of the afocal element. In this fifth aspect of the invention,the aperture, afocal element, and at least one mirror together form acommon optical path for the radiation from the source and to thedetector.

The foregoing may represent a description or definition of the fifthaspect or facet of the invention in its broadest or most general form.Even as couched in these broad terms, however, it can be seen that thisfacet of the invention importantly advances the art.

In particular, this facet of the invention can be used to apply theabove-mentioned steering precision and speed in the context of an activeoptical system—for example a lidar ranging system.

Although the fifth major aspect of the invention thus significantlyadvances the art, nevertheless to optimize enjoyment of its benefitspreferably the invention is practiced in conjunction with certainadditional features or characteristics. In particular, preferably theoptical system further includes means for operating the system in alaser application for communications.

In preferred embodiments of its sixth major independent facet or aspect,the invention is an optical system dynamically determining associatedangular direction throughout a specified range of angular directionsthat defines a field of regard of the system, of an external article ina volume outside the system; the optical system includes a radiationsource, an optical detector, an entrance aperture, and an afocalelement—associated with the aperture—reducing the field of regard of theexternal article and the volume as seen by the source and detector.

In this system each mirror of the at least one mirror has dimensions ina range not exceeding five millimeters. The foregoing may represent adescription or definition of the sixth aspect or facet of the inventionin its broadest or most general form. Even as couched in these broadterms, however, it can be seen that this facet of the inventionimportantly advances the art.

In particular, this system provides very fine pointing precision,arising from the reduction at the afocal element—but achieves this in asystem having modest aperture.

Even though this sixth facet of the invention thus significantlyadvances the art, nevertheless to optimize enjoyment of its benefitspreferably the invention is practiced in conjunction with certainadditional features or characteristics. In particular, preferably thisfacet of the invention is advantageously refined further by applicationof any, or most, of the preferences introduced above for other aspectsof the invention.

In preferred embodiments of its seventh major independent facet oraspect, the invention is an optical system dynamically determiningassociated angular direction throughout a specified range of angulardirections, of an external article in a volume outside the system; theoptical system includes a radiation source, optical detector, andentrance aperture—but no afocal element. It does include system at leastone mirror rotatable about plural axes, causing the source and detectorto address varying portions of the volume outside the optical system.

Each mirror of the at least one mirror has dimensions in a rangeexceeding five millimeters. The mirror has sufficient angular-deflectionrange that the external article receives radiation from the source andreturns the radiation to the detector throughout a field of regard equalto at least sixty degrees, notwithstanding absence of an afocal element.

The foregoing may represent a description or definition of the seventhfacet of the invention in its broadest or most general form. Even ascouched in these broad terms, however, it can be seen that this facet ofthe invention importantly advances the art. In particular, this systemachieves most of the objectives of other invention facets discussed inthis document—more specifically, all the objectives except the finerpointing precision associated with an afocal element—in a less-complexand less-expensive apparatus.

Here the cost and complexity of an afocal element are avoided.Additional details of such a system are presented in a later section ofthis document.

Although the seventh major aspect of the invention thus significantlyadvances the art, nevertheless to optimize enjoyment of its benefitspreferably the invention is practiced in conjunction with certainadditional features or characteristics. In particular, preferably themirror has sufficient angular-deflection range that the external articlereceives radiation from the source and returns the radiation to thedetector throughout a field of regard equal to ninety degrees, in atleast one rotational axis.

In preferred embodiments of its eighth major independent facet oraspect, the invention is a passive optical system—having no radiationsource to initiate return from remote objects—dynamically determiningassociated angular direction (throughout a specified range ofdirections) of an external article in a volume outside the system. Theoptical system includes an optical detector, entrance aperture, andafocal element.

The afocal element is associated with the aperture. It enlarges thefield of regard of the external article and the volume as seen by thesource and detector. Also in the system, disposed along an optical pathbetween (1) selectively, the source or detector and (2) the entranceaperture, is at least one mirror rotatable about plural axes and causingthe detector to address varying portions of the volume outside theoptical system. Each mirror of the at least one mirror has dimensions ina range exceeding five millimeters.

Due to the enlarging of the field of regard together with rotation ofthe at least one mirror, and substantially without changing magnitude ofthe enlarging, the external article receives radiation from the sourceand returns said radiation to the detector throughout the specifiedrange. The aperture, afocal element, and at least one mirror togetherform a common optical path for said radiation from the source and to thedetector.

These and other principles and advantages of the invention will be fullyunderstood from the following description of preferred embodiments,considered together with the accompanying illustrations, of which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective or isometric view of an operationaltwo-axis-controllable prototype octagonal servoed-steering-mirrorassembly—a diagram of a fast scanning mirror (FSM), which we sometimescall a “mini-scan-mirror” (MSM);

FIG. 2 is a graph of flux density vs. electrical power, in watts,through electromagnetic coils in certain embodiments of our invention;

FIG. 3 is a diagram, highly schematic, of a new alternativeservoed-steering-mirror FSM design that has an added ball joint forsuspension—with outboard circles representing regions of high magneticfield in the precursor (i. e. current) design, and inboard circlesshowing regions of newly increased magnetic field in this design;

FIG. 4 is a set of views of two-axis FSM or “MSM” mirrors that weconsider best implemented with jewel bearings, or variant refractorybearings—

-   -   the “A” view being in perspective or isometric, taken very        generally from the “front” side (the radiation-beam steering        side), with the gimbal-like steering elements rotated        out-of-plane for a clearer view of the several parts—and for a        prototype octagonal configuration,    -   the “B” view being likewise from the front and for the same        device but with the steering elements rotated into an        overall-flat orientation,    -   the “C” view being analogous to the “B” view but taken from the        rear,    -   the “D” view being a group of three mechanical drawings showing        the same device from above (left-hand subview), side (central        subview) and rear (right-hand subview) indicating dimensions in        millimeters,    -   the “E” view being a drawing like the “A” and “B” views, but        “exploded”, of a steering module using the same prototype        mirror,    -   the “F” view being a view like the “A” and “B” views, but        showing only the prototype-mirror subassembly (made up of an        octagonal mirror together with intimately associated support and        rotation elements), individually,    -   the “G” view being a like view of a prototype inner        support/steering ring, also usable for a production model,    -   the “H” view being a like view of a prototype or production        outer ring, and    -   the “I” view being like the topmost views in the “E” view, but        for a variant mirror having reflective surfaces formed directly        on both sides of the magnet, and an axle or axles attached to        the magnet directly;

FIG. 5 is a pair of drawings—“A” and “B” views like FIGS. 4A and 4Irespectively—but for a circular production mirror rather than theoctagonal prototype;

FIG. 6 is a pair of drawings showing an electromagnetic drive for theFIG. 4 production mirrors—the “A” view being a conceptual sketch showingoperational principles; and the “B” view being generally like FIG. 1B(but for a production system);

FIG. 7 is a longitudinal-section (i. e., taken in a plane along thecentral axis) of a “vee”-style (i. e., “V-shaped” in longitudinalsection) jewel bearing;

FIG. 8 is a perspective view of two stainless-steel pivots for use withthe FIG. 12 bearing;

FIG. 9 is an elevation of a ring-style bearing, shown with a shaft andan end stone—with the ring itself in longitudinal section;

FIG. 10 is a set of mechanical layouts—namely one common front view andseven different side views in longitudinal section—for jewel (e. g.sapphire) bushings having seven corresponding different shapes, namely(from left): straight hole, “olive” hole (having a curved longitudinalsection), “bombé” face (having a curved cross-section) with straighthole, bombéface with olive hole, oil-cup face with olive hole, oil-cupface with straight hole, and double-cup faces with straight hole,respectively;

FIG. 11 is a graph of mirror acceleration vs. mirror thickness inmeters, for driving current of one ampere is through 200 windings;

FIG. 12 is an electrical schematic, somewhat conceptual, of a circuitfor closed-loop control of a scan mirror—the schematic being juxtaposedwith a drive-apparatus drawing like FIG. 6B;

FIG. 13 is a basic conceptual diagram of three-dimensional geometry fora steerable mirror as shown in FIGS. 1 through 10;

FIG. 14 is a like diagram indicating how mirror angular position ismonitored by an embodiment of our invention that uses an areal detectorand a reflector at the back surface (not used for beam steering) of asteering mirror;

FIG. 15 is a like diagram indicating how mirror angular position ismonitored by an alternative embodiment that uses a pair ofretroreflectors at the back surface of the mirror, with afringe-counting laser interferometer;

FIG. 16 is a like diagram showing detail of the FIG. 15 interferometer;

FIG. 17 is a conceptual perspective representation of an operationalimplementation of a roving foveal sensor, capable of simultaneouslygenerating a wide-FOV (or wide-FOR) image and a narrow-FOV image (seeinset view)—both at high resolution—with the corresponding narrow-FOVimage inset into the wide-FOR (or wide-FOV) image, making a compositeoutput view; and further showing a preferred embodiment of the inventioncollecting those two images; and more specifically the drawing showsgeneral-aviation collision avoidance;

FIG. 18 is an optical diagram, somewhat conceptual, of the roving fovealdesign—one of three configurations that each include a beam splitter forpicking off a large-FOR “staring” detector from inside an afocal opticalassembly; is but here more specifically the drawing showing a preferredconfiguration in which a narrow-FOV image is movable with respect to thewider-FOV or FOR image, for viewing, so that an inset narrow-FOV imagecan be placed—relative to the wider image—as preferred by an operator orsystem designer, and indeed can be moved about, in relation to the widerimage, in real time;

FIG. 19 is an exemplary optical design schematic of a color-correctedafocal optical system according to a preferred embodiment of theinvention, for operation with visible light;

FIG. 20 is an operational flow chart, highly schematic and showingmethods—according to our invention—that also make use of MEMS mirrors ofthat same new generation;

FIG. 21 is a block diagram, with most portions symbolically in sideelevation but certain other portions (an aperture-lens assembly 14 and alens/detector assembly 22) symbolically in isometric projection, of abasic first function—namely, a detection function—for preferredapparatus embodiments of the invention; this drawing, together withFIGS. 22, 23 and 25, shall be interpreted for purposes of this documentas representing either an enlarging-only afocal system or areducing-only afocal system, or an afocal system selectively capable ofeither operating approach;

FIG. 21A is a like diagram, but having no afocal element at all—andcapable of operation over a relatively large field of regard solely byvirtue of operating a steering mirror over a sufficiently largemechanical-deflection range to achieve a sufficiently largeoptical-deflection range;

FIG. 22 is a like diagram showing an extension of the preferredapparatus embodiments to encompass a second function, namely opticalanalysis;

FIG. 23 is another like diagram but now showing a further extension toencompass dual forms of yet a third function, namely response;

FIG. 24 is a multiapplication block diagram representing apparatus andprocedures, using the apparatus embodiments of FIGS. 21 through 23 forthe above-mentioned and other functions, and in a number of variegatedapplications;

FIG. 25 is a diagram generally like FIGS. 21 through 23 but with thelens and detector assemblies 14, 22 enlarged for presentation ofdetails;

FIG. 26 is a diagram conceptually representing a spiral-scanning rasterpattern for use in any of the FIG. 21 through FIG. 25 systems andmethods;

FIG. 27 is a detailed top plan, somewhat schematic, of a prior-art MEMSmirror assembly with integral gimbal system—and showing direction of amagnetic-excitation field (after the “prior art” FIG. 2 in theabove-discussed '921 patent of Bernstein, Taylor et al.);

FIG. 28 is a simplified view of the same assembly, together with magnetsfor imposing such excitation (after “prior art” FIG. 3 in the samepatent);

FIG. 29 is a like view, but of a first part of an embodiment of theinvention that is taught in the '921 patent (after FIG. 8A in the samepatent), having separate “X-axis control coils” on the mirror pad, atopposite sides of the horizontal flexure and associated rotationaxis—and is prior art with respect to this present patent document, butnot with respect to the '921 patent;

FIG. 30 is a like view of a second part of the same embodiment (afterFIG. 8B in the same patent), but instead having separate “Y-axis controlcoils” on the mirror pad, at opposite sides of the vertical flexure andassociated axis—likewise prior art with respect to this document but notto the '921 patent;

FIG. 31 is a system diagram, highly schematic, showing aspects of ourinvention that incorporate one or more MEMS mirrors of the “newgeneration” discussed in the “BACKGROUND” section of this document (seerelated notes following this list);

FIG. 32 is a like diagram of two representative FIG. 31 mirrors in anend-to-end array and with the FIG. 31 base, but now seen edge-on andparticularly shown with the mirrors rotated away from the common baseangle (here forty-five degrees) by an arbitrary, illustrative amount(roughly nine degrees); and further with one of the mirrors adjustedalso in piston, to a position 112′ as shown in the broken line, forreasons explained herein;

FIG. 33 is a plan view of a magnetic circuit used in certain of our FSMcontrol-system experiments described in this document;

FIG. 34 is a simplified “electrical” model of the FIG. 33 magneticcircuit;

FIG. 35 is a graph of flux density generated by our FIG. 33 FSM magneticcircuit, as a function of current;

FIG. 36 is a top plan view of the mirror, together with one “mu-metal”arm used in the same magnetic circuit;

FIG. 37 is a pair of diagrams, quite schematic, defining some parameters(magnetic field, coil length and coil circumference, of a magnetic-fieldcoil with an air core—used in certain variants of the FIG. 33 circuit;

FIG. 38 is a like diagram defining relationships between magnetic momentand torque, for the same circuit;

FIG. 39 is a graph of the torque developed at the mirror, as a functionof number of ampere-turns in the driving coil;

FIG. 40 is a plot of resulting response times for a 10-mm mirror andmagnet (with no magnetic-circuit ring around the magnet), as a functionof magnet thickness in meters;

FIG. 41 is a perspective view of a prototype of the FIG. 33 magneticcircuit, assembled to our experimental structure—with a Hall-effectsensor under the intended position of the mirror;

FIG. 42 is a like view of the prototype on our experimental bench withelectronic modules ready for use;

FIG. 43 is a set of five views of diverse prototype magnetic circuitsused in the FIG. 42 experiments; and

FIG. 44 is a graph of flux density vs. electrical current, in amperes,through the coils.

FIGS. 1 through 5, 7 through 10, 12, 18, 19, 33, and 41 through 43 arevery generally to scale.

In FIG. 31, for the sake of simplicity, mirror control coils andresulting forces are explicitly illustrated for only a single axis ofrotation. It is to be understood that the coils and forces are closelyanalogous for the orthogonal direction, just as illustrated in FIGS. 29and 30 respectively. Preferably, however, one of the rotationaldirections is managed by use of a MEMS or FSM steering frame (not shown)surrounding the mirror pad and carrying coils for driving in thatdirection, as shown in FIG. 27 and discussed in the above-mentionedpatent documents of Bernstein and Taylor. If desired, dual-axis rotationof a single mirror-carrying element can be substituted, as alsodescribed in those documents. FIG. 31 is meant to represent both thesekinds of dual-axis implementation, and also several other systemvariants as more fully detailed below.

Thus portions of FIG. 31 are representative of a singleelectromagnetically controlled mirror 311 with a rotational axis 317; orequally well of an end-to-end two-mirror array 311, 312 withspaced-apart rotational axes 317, 318; or also equally wellrepresentative of a side-by-side two-mirror array 311, 314 with axis317—or 312, 315 with like axis 318, etc.—or of a larger array such asthe six-mirror assembly 311-316 expressly shown. In each case the coilse. g. 311 c, 311 d, in cooperation with magnets that may e. g. be in thebase 319, provide respective forces 311 a, 311 b or 312 a, 312 b forrotation of the respective mirror(s) 311 etc. about the correspondingaxis or axes and flexure(s) 317.

FIG. 31 also includes an auxiliary optical system 331-339 for purposesrelated to optimizing imaging sharpness, as detailed below. The drawingfurther includes a generalized element 351, which represents any ofseveral supporting or utilization devices, or combinations of suchdevices, that are advantageously incorporated into the system. Thepossible devices are enumerated and discussed in following sections ofthis document.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Preferred embodiments of our invention provide an optically agileimaging system, with related methods, based upon certain active opticalcomponents—in combination with a servoed-beam-steering subsystemaccording to the coowned patent documents listed earlier. The resultencompasses a small, lightweight and relatively economical sensortechnology.

This technology is most typically, but not necessarily, for use withsmall unmanned vehicles—whether aerial or otherwise—as well as mannedcraft and indeed stationary installations. Preferred embodiments of theinvention have several practical applications mentioned earlier.

The resulting capabilities enable e. g. commercial air traffic systems,astronautical operations and military agencies to enjoy very greatlyenhanced but low-cost imaging—including persistent surveillance andpointing. Advanced imaging, using electronically addressable pointingand imaging capability without a conventional gimbal, enclosing theentire optical system, provides the basis for lightweight, low-power,reliable imaging performance.

OPTICAL SYSTEM WITH FAST SCANNING MIRROR (“FSM”)—This invention providesa small, low-friction steering mirror that can scan, very rapidly in twoorthogonal directions—on jewel bearings or equivalent. In performanceterms, for most comparisons, equivalents to jewel bearings includebearing surfaces made of ceramic and other refractory materials.

For purposes of this document, the term “refractory” adheres toconventional definitions in engineering etc., namely materials that aredifficult to work, or that are extremely hard, or sometimes that have ahigh melting point or are “difficult” to melt. Accordingly, refractorybearings include, without limitation: ceramics, cast materials such asused in making tiles and furnace linings, some types of concrete orcement, gemstones, synthetic gems (including original Hart artificialdiamonds), and other tough minerals. Related classes of potentiallyuseful materials are refractory plastics, known in the marketplace, andrefractory metals such as tungsten, molybdenum and alloys. Most suchplastics, however, at present are inadequately stable for ourpurposes—particularly under heat or relatively high stress.

Most such materials can be cast or otherwise formed by varioustechniques. Vapor deposition appears particularly promising.

Current-day websites with details about low-friction coatings include,among others—

Composite Diamond Coatings:

-   -   enduracoatings.com/prod1100f.asp    -   enduracoatings.com/prod1100b.asp

Thin-Film Coatings:

-   -   pvd-coatings.co.uk/MoST-coating.htm    -   industrialcoatingsworld.com/Low-Friction-Coatings/Vapor-Deposition-Low-Friction-Coatings.html    -   industrialcoatingsworld.com/Low-Friction-Coatings/Dry-Film-Lubricants-Low-Friction-Coatings.html

and:

-   -   suncoating.com/molykote.html.

(As will be understood, in a web browser any one of these URLs must beentered all in one continuous line, e. g. with no line break between“Coatings/” and “Vapor”.)

For example, the PVD site discusses molybdenum disulphide, said to have“an ultra low coefficient of friction (0.01-0.06)”. As another example,the “EnduraCoatings” links relate to composite diamond coating.

In this patent document it will be seen that friction is a veryimportant and rather complicated topic for best practice of certainembodiments of our invention. For full enabling disclosure of the bestmodes of practice, that topic will be taken up in considerable detail,in succeeding sections of this document.

Scanning is very fast, because the control bandwidth in one or bothrotational axes is on the order of twice that attainable with proposedsystems of Bernstein (Draper Corporation), which are based on monolithicsilicon construction and have unitary torsional springs as flexures.Hence our steering mirror can be used for extremely rapid and accurategeneration of an optical raster, or object tracking.

Furthermore if desired this invention can be implemented with zerorotational restoring force, so that rotational position tends to bequite stable without application of large continuing positional force.For maximum excursion we prefer to drive the mirror magnetically asdetailed below; however, other rotational drive arrangements are readilyavailable.

In some preferred embodiments the mirror has a permanent magnetic fieldalong the Z axis (FIGS. 12 through 15, and 37). This field facilitatesrotating the mirror about the local X and Y axes by application ofcurrent to nearby electric coils, resulting in respective magneticfields. The fields interact with the permanent Z-axis field to createtorque, impelling the mirror about the X and Y axes.

The one or more electromagnetic coils 12 (FIGS. 1 and 12), 21 (FIG. 6B),412 a-d (FIG. 33) are switched or modulated (or both) to createattractive and repulsive magnetic forces that result in a net torque onthe scan mirror (FIG. 38). Switching between positive and negativevoltages reverses current through the coils, providing magnetic field ofdesired direction. Additionally, one or more coils can be simultaneouslyswitched to a “brake” terminal B that electrically completes the coilconduction path—resulting in back-emf that brakes the rotating mirror,e. g. about q_(X).

The steering module (FIGS. 4A through 4E, and 5A), allowing for rotationin two axes, comprises three major components: the mirror subassembly11, 16, 18, 25 (see also FIGS. 4F, 4I and 5B), inner ring 15, 14, 19, 15(see also FIG. 4G), and outer ring 13, 17, 24, 19, 27 (see also FIG.4H). The mirror subassembly rotates on two opposing pivot points 16, 19inside the inner ring. Together, the mirror subassembly and inner ringrotate about another set of pivot points 14, 19′ between the inner andouter rings and mounted at right-angles to the pivot points between themirror subassembly and inner ring—thus providing independent orthogonalrotations of the mirror. The mirror subassembly is driven in rotation bya set of four or more electromagnetic coils 12, 21, 412 a-d mountedaround the perimeter of the steering mirror as noted above.

In the Japanese inventions mentioned earlier, the angular range is verysmall, at best three degrees on only one axis. In comparison thepreferred jewel-bearing and equivalent embodiments of our inventionare—by design—able to achieve mechanical-excursion angles exceeding 25degrees in both axes. A jewel-bearing design also provides largeapertures, from seven millimeters to three and even five centimeters,which are not practical with the silicon torsion-flexure design. Ourinvention contemplates still-higher apertures, to the order of e. g. tencentimeters and more, without major modifications in designconsiderations or scaling techniques.

MIRROR ASSEMBLY—In our preferred prototype embodiments the mirrorsubassembly 25 (FIG. 4E) comprised a ten-millimeter octagonal mirror 11of silica, a seven-millimeter grade-N50 neodymium magnet 18, and an axle16 made of 300-series stainless steel—bonded together with epoxy.

Because the center of gravity (CG) was not located through the center ofthe axis of the pivot points 16, this configuration (although quiteexcellent in comparison with all known prior art) was not ideal.Accordingly, the production version of the mirror subassemblycontemplates simplifying the design by combining the mirror and magnetinto one piece (FIGS. 4I, 5A, 5B). In this preferred design theproduction mirror consists of a magnetic material (grade-N50neodymium)—plated and then polished to an optical-grade mirror finish,optionally on both sides. (Being symmetrical, such a mirror cannot beinstalled “upside down”.)

This design allows alignment of the pivot points with the magnet CG. Theback of the mirror, in this configuration, can be used as part of anoptical-position sensor for the control feedback loop.

In such a design the mirror pivots form a nonmagnetic axle that rides ina bearing 19 (FIG. 4A). The axle pins 16 can be made part of the mirrorassembly 25 and the bearings 19 part of the inner ring 15 (FIGS. 4A. 4B)or vice versa, as preferred.

(In purest principle it would even be possible to make one of the axlepins 16 part of the mirror assembly 25 and the mating bearing 19 part ofthe inner ring 15—but then to reverse these allocations for the oppositeone axle pin 16, i. e. making it part of the inner ring 15, and itsmating bearing 19 part of the mirror assembly 25. Such an arrangementmight function poorly because of frictional or weight imbalance betweenthe two edges of the mirror assembly. Analogous considerations apply tothe axle pins 14 and mating bearings 19′ at the outer ring.)

Alternatively the pivot points can be attached to an additionalintermediate structure. The mirror/magnet can be mounted to thatadditional structure.

It will now be seen that preferred embodiments of our invention yieldseveral important benefits: increased apertures that MEMS devices cannotprovide, translational stability in X, Y, and Z axes that prior “bigmirrors” do not provide, no requirement for isolation in high vibrationenvironments, reduced power demand to drive the mirror, and very greatlyreduced weight relative to other steering systems. Especially criticalis the much higher bandwidth for step response.

In the current state of our FSM development, the natural resonantfrequency of our mirror/magnet assembly is roughly two kilohertz. Thedriven response bandwidth is typically one hundred to two hundred hertz,but much higher (e. g. one kilohertz) with refined control-system designand components.

It is also very clear that the response bandwidth as driven can be madefar higher still, with advanced electronics not yet available. Such newcomponents will provide, for example, a faster CPU clock or A/D clock,or both. In addition the apparatus will preferably operate with highermagnetic fields.

For apertures below some 5 mm, such FSM embodiments are not usuallyapplicable—but for fine diffraction control this limitation is notordinarily meaningful. Also, consistency in the assembly process, asbetween devices, is not quite as good as in monosilicon MEMSconstructions, made by is techniques similar to those used inintegrated-circuit lithographic techniques.

INNER RING—The inner ring plays a key role in the design. In ourprototype it is an aluminum ring approximately 12 mm in diameter and 2mm thick. It also contains pivot points for both the mirror subassemblyand the inner ring itself, allowing the mirror to rotate about bothaxes.

As with the mirror subassembly, the inner ring shown is a prototype. Wemay elect to change the shape and pivot configuration for the productionversion—for example to provide spring preload for the pivot points. Thering can be made of nonmagnetic material such as an aluminum alloy,beryllium, beryllium-copper, titanium alloy, tungsten, 300-seriesstainless steel, silica or other suitable substance.

OUTER RING—The outer ring 13 (FIGS. 4E, 4H) has two primary functions:it mounts the overall steering-mirror assembly to the base structure 23(FIG. 1B), and holds one side of the pivots for the inner ring. In theproduction version we will either enlarge it or remove it completely, sothat the electromagnetic coils 12, 21 can be mounted closer to themirror assembly and thereby increase the torque that the electromagnetscan apply to the mirror assembly. FIGS. 7 through 10 are after currentwebsites at industrialjewels.com, particularly pages vee_jewel.htm,ring_jewel.htm, swissjewel.com/sapphire_ring_jewel_stock_sizes.htm; andsmallparts.com/products/descriptions/vjpx.cfm—dimensions in the latterbeing stated in millimeters.

PIVOT POINTS (JEWEL BEARINGS)—Two suitable jewel-bearing configurationsare the “vee” (FIG. 7) and “ring” (FIGS. 9 and 10). Our prototypeincorporates a vee bearing for both pivots; however, ring bearings aremore rugged and may be preferred for final construction. The veeconfiguration uses nonmagnetic stainless-steel pivots (FIG. 8) that ridein vee bearings. These various components are used in precisioninstruments such as analog ohm/voltmeters and compasses, but neverheretofore in steering-mirror systems.

The ring configuration has a steel shaft that rides in e. g. aring-shaped sapphire bushing 119 a (FIG. 9) and is held in place axiallywith a sapphire endstone 119 b. If the bearing assembly will be used ina high-shock or -vibration environment the endstone is spring loaded.The latter arrangement is commonly used for light-shock applications; asilicone cushion, for high-shock loads up to 1000 g's. A ring jewel withan “olive” hole 142 (FIG. 10, hole with radius) offers lower friction.The radius surface of the hole provides minimum contact with the pivot,and thereby minimum friction.

From the foregoing discussion of our fast scanning-mirror (FSM) details,we now turn to details of our two-axis feedback sensors.

TWO-AXIS TIP/TILT SENSOR—We prefer to use a local tip/tilt sensor, whichmeasures q_(X) and q_(Y) position, velocity or acceleration, in anegative-feedback control loop 52 (FIG. 12). This local feedback iscombined with inertial feedback 53, and subtracted from the desiredangular position, velocity or acceleration command 51.

For each rotational degree of freedom, this difference is provided tothe controller—a position, integral, derivative (“PID”) or othersuitable controller. Resulting electromagnetic-coil commands (currents)58 are then provided to the coils 12, driving the mirror by magneticallyinduced torque.

Preferred embodiments of our invention contemplate twoapproaches—primarily alternatives—for a sensor to measure changes inangle about the two rotational axes of a mirror. In a first approach,access to the back of the mirror is required, but the front(beam-steering reflective surface) of the mirror is left unobscured.

The mirror can rotate 62, 61 about two nominally orthogonal axes, X andY (FIG. 13), relative to the front reflecting surface whose normal is Z.According to the first approach, a laser source 63 (FIG. 14) is directedtoward an auxiliary reflecting element 111′ located on the back surfaceof the mirror 111, i. e. the “−Z” side. This element reflects thelaser-source beam 63 along a path 64 toward a two-dimensional (“2-D”)areal detector 65.

This detector could be a position sensing detector, a quad detector or adetector array with multiple elements. The areal detector 65 measuresthe location (x′, y′) of the reflected laser beam 64 onto a planeparallel to the original nonrotated X-Y mirror plane X′, Y′. As themirror rotates about the X and/or Y axis the angular rotation aboutq_(X) and q_(Y) can be estimated, given knowledge of the (x′,y′)detector measurement and knowledge of the distance Z′ from the mirror.The angles about the axes are respectively calculated by defining:angle1=arcsin(x′/r) andangle2=arcsin(y′/r*cos(angle1))where r=sqrt(x^2+y^2+z^2)—or to the first order if we have small anglesand the PSD is “far” away we can reduce this to arcsin(x′/Z′) andarcsin(y′/Z′). In this regard, the distance of the mirror reflectivesurface from the center of rotation establishes the denominator for asine or cosine effect.

The second preferred measurement approach uses a retroreflectorinterferometer pair 111 a, 111 b (FIG. 15) that measures the change inrelative distance Z′₂, Z″₁ between fringe-counting interferometers 65 a,65 b and the retroreflectors 111 a, 111 b respectively. Theretroreflectors are attached to the back of the mirror 111; and theinterferometers, to a fixed base (not shown).

By tracking the change in distance between the retroreflectors (Z″₁−Z″₂)and knowing the relative distance Y″₁₋₂ between elements 65 a, 65 b ofthe interferometer pair, this system finds the rotational angle q_(X)about the X axis—as equal to arctan((Z″₁−Z″₂)/Y″₁₋₂. Similarly a thirdretroreflector interferometer set (not shown) that is not in line withthe first pair 65 a, 65 b can be used with one or both of those othersets to measure rotation q_(Y) about the Y axis.

A three-set configuration can also be used to measure displacements, inZ, of the plane containing the three retroreflective elements 111 a, 111b. The retroreflective elements can include a sphere or cubical corner,which reflects entering light to leave the reflector parallel to theincoming light-propagation vector.

Each fringe-counting interferometer 65 a, 65 b, preferably aTwyman-Green configuration (FIG. 16), includes a laser 63 that directs abeam 63 a to the front side of a splitter 67. Part of that beam 63 a isreflected upward along a first path 69 a to the retroreflector 111 a or111 b, where that part of the beam is returned downward along a parallelpath 69 b, this time back through the splitter, to a detector 68. Theremainder 63 a′ of the original beam 63 a is reflected from the fixedmirror 67, along a return path 64 a, 64 b to the back side of thesplitter 67 and then downward along a path 69 b to the detector.

Depending on phase between the two beams 69 a, 69 b, constructive ordestructive interference occurs—modulating the recombined beam to thedetector 68 and thereby the detector output electrical (or other) signal70.

In summary the interferometers measure change in Z, Z″ etc. based onknowledge of the laser 63 wavelength—and based on counting fringes aslight propagating along the moving paths 69 b reflected from theretroreflectors 111 a, 111 b interferes with the fixed-path returns 64a, 64 b from the fixed mirrors 67 in the interferometers.

With this approach it is possible to measure relative changes, or—byusing two or more wavelengths—to measure the absolute distance. Thislatter possibility is an example of an approach that uses just oneinterferometer.

BERNSTEIN/DRAPER APPROACH—As noted earlier, our jewel-bearing fastscanning mirror is in general significantly superior to MEMS mirrorsformed from monosilicon components with torsional flexures. Bandwidthmeasured for step response with jewel bearings has proven to be at leasttwice as high as for the MEMS devices. If monosilicon and torsionalflexures are nevertheless preferred, a relatively simple process can beused to fabricate our servoed MEMS mirrors, requiring only three photosteps.

The starting material is a thick silicon-on-insulator (“SOI”) waferlayer—on a 300 μm handle wafer, separated by oxide 1 μm thick. A metalmirror is deposited by sputtering, followed by liftoff. Inductivelycoupled plasma (“ICP”) etching steps define the mirror, torsionalsprings, and monosilicon gimbal. A second plasma-etching step removesthe handle wafer so that the mirror can move.

The thickness of the springs and mirror is established by the thicknessof the starting SOI material. A magnet (preferably a permanent magnet)inserted in a cavity in the silicon mirror enables light-deflectingactuation about two axes by a set of four drive coils.

Positional feedback sensing for the mirror comes from the output ofsensors such as PSDs, as described above, so that the deflection angleis determined by monitoring mirror-control signals. Sensitivity for thisfeedback is related to the operating principles of both the sensor andthe above-mentioned mirror drive magnet. Full understanding of thefeedback-sensitivity subsystem is a primary determinant of potentialoverall system performance.

There is some risk that for some imaging applications these MEMS-typemirrors may not perform as expected. For example, they may not rotate asfar as desired or may be too fragile for a high-shock and -vibrationenvironment—even after flexure optimization. Additionally, feedbacksensors may not give sufficient positional accuracy for adequateclosed-loop mirror control. As noted elsewhere in this document, suchtorsional-flexure mirrors (especially when large) are subject to bendingout of plane; such distortion in turn leads to beam divergence and“splatter”—i. e., waste of substantial fractions of the beam power thatare misdirected away from the nominal beam direction. To address theserisks, we recommend initially fabricating a 10-mm (or larger) servoedmirror and fully characterizing its performance. Particularly advisableis demonstration of hemispherical pointing in both open- andclosed-control-loop environments.

Also highly recommended is characterizing the sensor performance tooptimize mirror positional feedback. Additionally, step-stare capabilityshould be demonstrated in the laboratory to characterizeimaging-stability requirements. Based on these performance results,design changes can be developed for production servoed mirrors.

Several improvements have already been considered in anticipation ofpotential problems in our current most-preferred servoed-beam-steeringdesign. It is a major departure from earlier embodiments relying on the“large mirrors” patent document noted above or on theBernstein/Draper-style mirror concepts.

In the latter context for example, a minor design change cansimultaneously impact vibration and shock concerns, along with sensorfeedback sensitivity and accuracy. In those earlier embodiments, simplyplacing a hole in the permanent magnet that was mounted behind themirror provided for the possibility of including ball-joint suspension(FIG. 3) while simultaneously increasing the magnetic field near thesensors.

Here regions marked by outboard circles represent relatively lowmagnetic fields in such precursor designs, whereas the inboard circlesshow how the magnetic field increased near the sensor—leading toincreased mirror-position accuracy for some sensor types. Because of theball-joint suspension, this design might have required an alternativesensor approach: the ball-joint post might have interfered withinstallation of some types of mirror-position sensors.

Essentially all these concerns disappear or are very strongly mitigatedin our current jewel-bearing, nonmonosilicon, and nonMEMS embodiments.We recommend careful consideration of all recognized alternatives.

CONTROL-SUBSYSTEM CONSIDERATIONS—Demonstrating performance at the systemlevel entails tests of so-called “persistent surveillance tracking”(step-stare and platform dynamics) with INS (inertial navigation system)feedback. Such investigation can be conducted in the laboratory withsimulated platform motion. Specifically, it is helpful to integratesignals from mirror-position sensors with inertial data.

One low-risk approach to achieving this goal is to first establishopen-loop control of the mirrors (i. e. without using mirror-feedbacksensors). Hemispherical beam steering can then be demonstrated, withability to project a beam through a commercial wide-FOV afocal lens toachieve the hemispherical beam steering, through integration of a simplevisible laser.

The next objective should be closing the pointing control loop. In alaboratory environment with the system mounted to a stationary platform,closed-loop control and stability can be characterized easily.

A final step can be incorporating inertial-sensor feedback, todemonstrate closed-loop stabilization relative to an inertial referenceframe. Mounting the system on a platform with known vibration dynamicscan facilitate obtaining full characterization of the inertiallystabilized system.

Although it is a salutary design goal to achieve mirror-pointingprecision better than 0.1 mrad, this objective may overconstrain theproblem—in the sense that inertial stabilization errors will be acombination of the mirror-pointing stability plus the INS performancecapability. Typical INS systems used on small UAVs (unmanned airbornevehicles) tend to have attitude accuracies on the order of 5 to 7 mrad,and update rates of roughly 50 to 100 Hz. Such issues should bethoroughly investigated during development.

MIRROR SIZE, AND APPLICATIONS—Also as noted above, these steering-mirroradvances include development of much larger mirrors than heretoforethought feasible for high-bandwidth, low-power and low-weight systemsneeding good mirror planarity, good directional controllability, andreasonable linearity of directional adjustments—particularly suchmirrors of the jewel-bearing and equivalent ceramic and other refractorytypes (as well as secondarily the etched monosilicon types). In ourjewel-bearing mirrors, again, we are perfecting units of size up to 2and 5 cm and even much higher. We believe that our current regime ofdesign techniques and scaling considerations will hold good to sizes onthe order of 10 cm, and larger. (Our original choice of 10 cm was inpart to make our prototype reduction to practice and follow-up moreefficient, by adapting prototype components purchased from Draper.)Beyond that range, other unsuspected influences may well enter intodesign efforts; however, use of mirror arrays, and our known principlesof array optimization for diffraction control, could extend the usefulsize range much farther than 10 cm.

As a practical matter, beam-deflector arrays or individual mirrorsexceeding 10 cm may not be of interest now. This is so because modulesof 10 and even 5 cm appear to fully satisfy current objectives, and verydifferent solutions are available where bigger systems are desired.Nevertheless it appears that larger units are fully functional;therefore they remain of interest for technical applications that maybecome more-interesting in the future. Such applications may relate e.g. to high apertures for high-energy response beams.

More generally our current inventive work emphasizes mirror types of amuch greater variety than in our earlier investigations. Those earlierefforts, to a large degree, pursued system designs using MEMS-typemirrors, as well as monosilicon etched planar structures with integraltorsional flexures (or “hinges”) serving as gimbals. Thus the presentinvention partly consists of introducing very large nonMEMS mirrors,nonplanar-silicon etched mirrors, and nontorsional-flexure steeringmirrors.

Other aspects of the present invention, however, include very differentapplications of these same newer large steering mirrors. These new anddifferent applications in particular include uses of these mirrors inlidar systems, and in other types of active optical systems (not to beconfused with active optical elements), and in many other kinds ofoptical systems e. g. based on null-balance operation of the mirrorsteering apparatus.

The invention is not limited, however, to active systems. As seenelsewhere in this document, certain aspects of the invention haveimportant applications in passive optical systems.

Many such systems, for instance, are systems for noticing and opticallyfollowing objects outside the optical system, as set forth in several ofour coowned patent documents listed at the beginning of this presentdocument. Some of those earlier systems include provision of a “return”beam, is directed toward an external object to dazzle its operators orassociated automatic equipment, or even to impair its structure orfunction; others include provision for spectral analysis of radiationfrom the noticed and followed external object; and still others are forimaging of a noticed and followed external object or group of objects.

Others of our earlier systems include an afocal element that enlarges orreduces the FOV or FOR, or both, to enhance those object-noticing and-following inventions. Thus our current mirror developments areespecially effective when plowed back into our earlier system intereststo further refine and advance such systems.

Also notable is that some very useful embodiments of our inventioninclude no afocal element. In particular, because our newestrefractory-bearing steering mirrors have such a very wide scanningangle, in most cases we are able to obtain adequate field of regardwithout an enlarging afocal element.

Even in such situations an afocal element can still be useful, operatedin a reducing mode, to achieve very valuable finer pointing precisionand thereby more precise steering. Merely by way of example, taking areduction ratio of 5:1 in an afocal element can refine the precision ofpointing from e. g. 100 μrad down to 20 μrad.

People skilled in this field will appreciate that there exist twodifferent approaches to using an afocal optic in a reducing mode:

(1) the optic can be used in such a mode exclusively, if for instancethe system designers and operators are confident that no enlarging usagewill be meaningful for their particular combination of circumstances,applications, etc.; and

(2) the optic can be used in both reducing and magnifying modes at will,e. g. in alternation, if the designers and operators see utility inproviding such a varying capability.

We accordingly believe that both these two approaches fall very properlywithin the scope of certain of the appended claims. Accordingly boththese types of afocal-element claims are in fact appended; moreover,notwithstanding any textual detail herein that may seem to suggest thecontrary, the accompanying drawings (particularly FIGS. 21 through 23,and 26) shall be interpreted as illustrating either enlargement orreduction, or both, as appropriate to support those claims.

(Analogous observations can fairly be made about using an afocal elementin an enlarging mode—i. e., exclusively enlarging, or both enlarging andreducing—are appropriate, depending on the circumstances.)

As to the wide scanning angle mentioned above, our fast scanning mirroris readily able to operate over a mechanical range of ±22½°, i. e. afull mechanical range of 45°. Further, since the optical deflection istwice the mechanical deflection, the effective optical range is±22½°×2=±45°, or full range of 45°×2=90° total.

In fact a considerably greater mechanical and optical range is entirelyfeasible. In many practical circumstances, however, exceeding aninety-degree overall range of deflection is not productive. In suchcircumstances, the beam from the mirror at extremes of anover-ninety-degree range is obscured by interfering objects—e. g. by thehorizon.

For purposes of both energy collection and diffraction control,maximizing the system aperture is very helpful. In this connection too,our invention can exploit a reducing afocal element, or a cosine effecton effective aperture.

As mentioned above, some preferred embodiments of our invention compriseoptical systems that are passive—meaning that they simply receive andanalyze an incoming external optical beam, without either:

-   -   (1) initiating or generating such an incoming beam by emitting        an outgoing beam at the outset; and    -   (2) returning a reply beam in response to the incoming beam.

As will be seen, some of such passive systems according to our inventionare closely related, functionally, to the active systems that arepresented in this document and in our earlier coowned patent documents.

All these advances are thus valuable, and are especially importantaspects of the improved mirror according to our present invention. Thepresent document, taking into consideration the several coowned (andwholly incorporated by reference) patent documents mentioned above, doesexplain how to obtain the desired response bandwidth, low-power andlow-weight modules with good mirror planarity, directionalcontrollability, and reasonable linearity of directional adjustments.This document establishes such aspects as parts of the presentinvention.

POTENTIAL EQUIVALENTS TO JEWEL BEARINGS—THE PREVIOusly discussed “jewelbearing” construction has been found excellent in increasing the dynamicrange and related frequency-response characteristics of ourbeam-steering deflectors. Recently, however, we have come to therealization that such advantageous construction does not depend upon useof actual jewels in the bearings.

Rather, we have found that entirely satisfactory bearings can beprovided using ceramic or other refractory materials coated, molded orotherwise formed. Although the cost of the jewel bearings is quitemodest, these alternative constructions are significant advancementssimply in that they expand the range of materials and structures thatcan be used for practice (and included in patent coverage) of ourinvention.

FAST SCANNING MIRROR BEARING CONFIGURATIONS—Configuration of thebearings used for a fast scanning mirror should not be restricted to theuse of jewel bearings. Many bearing types could be used depending on theapplication and mirror size. The single most-important property of thebearing evidently is low friction. The smaller the mirror, the moresignificant the friction becomes. Increase of mirror diameter requireslarger forces to move the mirror due to the increase in inertia. Theequivalent torque corresponding to bearing friction thus becomes asmaller portion of total torque required to move the mirror at highaccelerations. As the mirror size increases, the number of options forbearings increases. Some of the bearings that may be useful in FSMdesigns are described below; this listing is not intended to beexhaustive.

Static and dynamic friction should be considered in selecting a bearing.Low static friction (sometimes called “breakaway” friction or“stiction”) is required in applications where pointing stability isimportant.

It is very important to recognize, however, that ideal operation of fastscanning mirrors according to our invention does not result from thelowest possible friction. Preserving some small, nonzero amount offriction in the bearings actually stabilizes the mirror, and makes themirror easier to control, though this technique does use somewhat morepower.

The best friction level varies with practical applications contemplated,with bearing type, and with size and mass of the mirror, magnet etc. Anoverall objective is to find the most-favorable compromise betweenmirror stability and speed (i. e. bandwidth) of pointing or steeringresponse. Merely by way of example, for a fast scanning mirror (FSM)using our jewel bearings and their ceramic and other refractoryequivalents—at a mirror diameter of 10 mm as implemented according tothe present document—our measurements to-date indicate that such a bestcompromise results from a coefficient of friction within the range 0.1to 0.15 (dimensionless).

For other bearing types, such as e. g. air bearings or magneticbearings, considerations such as just outlined are likely to be almostwholly inapplicable. Thus for air bearings, static friction is likely tobe negligible—and friction varies strongly with rotational velocity ofthe mirror.

More specifically, in the air-bearing regime, friction typically isextremely low but varies (at least for high mirror-rotation speeds) withthe square of velocity. Accordingly very different parametric values andcontrol considerations typically obtain.

It is nevertheless true in general, regardless of bearing type, that asfriction decreases to approach zero, any differential-equation analysisusing a classical imaginary-number coordinate system tends to “blow up”.That is, the so-called “poles” and “zeroes” of such an analysistypically tend to line up along the imaginary-number axis—a mathematicalbehavior that in the physical world corresponds to uncontrollableinstability.

Fortunately such difficulties are overcome simply by very carefulintroduction of small amounts of friction. The particular amountsdepend, again, upon the type of bearing and the values of otherparameters found in the analysis.

CERAMIC BEARINGS—These are similar to jewel bearings, but their hard,low-friction surfaces can be formed into many shapes and bearingconfigurations. Thus ceramic bearings can be customized to meetparticular needs of a variety of applications.

LOW-FRICTION COATINGS—These coatings are very versatile, especially inbearings for larger mirror sizes, as they can be applied to almost anysurface, especially almost any machined surface. Low-friction coatingsare applied by dipping, spraying or—perhaps most promisingly—vapor-phasedeposition. Coating thickness can vary from 2 to 100 μm. A particularlyadvantageous construction may result from simply spraying or otherwisecoating such suitable ceramic, or other refractory materials, into ahole or some to other constraining structure. Properties of such a holeor other structure, and other details of refractories design and use,can be learned straightforwardly from vendors, for example in vendorwebsites such as identified above.

Frictional properties of commercial materials, even commerciallyavailable bearings, should nevertheless be carefully validated by actualmeasurement for use in the analytical frameworks discussed in thisdocument. Vendor data may be sufficiently accurate for e. g. simplecomparative purposes such as vendors anticipate—as between differentsubstances and different finished products—but this is not necessarilythe same thing as use of such data in predictive calculations.

AIR BEARINGS—Such bearings consist of an air flow, or air in a sealedchamber, that supports a shaft or other movable (particularly rotary)element. Air bearings have low breakaway friction, and very low frictionin general. They would be most straightforward and therefore mostappropriate for the outer stage of our mirror support. A bearing for theinner, gimbal-like stage would require an air supply to the bearing overthe outer rotation axis. An air bearing also requires large bearingsurfaces, which would restrict use at the inner stage because of thecorresponding increase in material and inertia.

Friction arising in air bearings calls for a different analysis than injewel and other-refractory bearings. When moving quickly through air orother gases, mirrors and mirror/magnet assemblies (and generally mostother articles) exhibit air friction that changes very strongly withvelocity through the gas. As mentioned earlier, dominant behavior isquadratic variation with velocity. Modeling systems with such variationis feasible and should be performed before attempting to build asteering mirror with air bearings.

MAGNETIC BEARINGS—Use of magnetic support bearings may be awkwardbecause of potential interference with magnetic rotation drive. Carefuldesign, however, could minimize this limitation.

MAGNETICALLY STABILIZED BEARINGS—Dr. Benjamin Joffe, in U.S. Pat. No.6,176,616, U.S. Pat. No. 6,093,989, U.S. Pat. No. 5,986,372, U.S. Pat.No. 5,524,499, U.S. Pat. No. 5,331,861 and related patents, teaches afamily of bearing configurations that can control bearing backlash,wobble, and to an extent friction. As above, there is a potential issuein interference with a magnetic drive; nevertheless, use of suchbearings in our steering-mirror systems may provide significantadvantages, particularly for larger mirrors.

FAST SCANNING MIRROR (“FSM”) DESIGN GUIDELINES—Several tradeoffs must beconsidered in the design of an FSM. The inertia of the moving parts hasa direct relationship to the torque required to meet dynamic scanrequirements. The acceleration of the mirror on each axis of motion isrelated to the difference between the generated B field and thepermanent magnet of the mirror.

Increase in thickness of the mirror magnet has a diminishing return(FIG. 11). The functions graphed in FIGS. 11 and 40 strongly imply thatmaximum torque and minimum response time call for an optimumthickness—generally about 2.3 mm—for the magnet/mirror assembly;however, it must be recognized that these graphs apply only for aparticular value (one centimeter) of diameter, for the assembly. Thisfact leads to a need for detailed consideration of scaling, which willbe introduced very shortly. As will be understood from such details,determining optimum geometries is moderately complicated—but can bebrought under control using the analytical framework presented in thisdocument.

Other tradeoffs are available in the mirror construction, particularlyas to the solid magnet or ring magnet. The ring magnet could be replacedby four individual magnets adjacent to the electromagnet cores. Benefitsof an increased number of coils to generate a stronger driving magneticfield, however, are counterbalanced by space, power demand, anddissipated energy at the coils, and application performancerequirements.

The drive mechanism is primarily useful for relatively small mirrorapplications, involving diameters e. g. below 5 cm. The drive efficiencyfor larger mirrors can be improved with more-conventional on-axis drivemotors. For reasons indicated below, the torque required to drive themirror at the same rate increases by, very roughly, the third power ofthe diameter.

Of particular interest in this regard are such inquiries as what happensto the inertia—and required torque, and magnetic-flux density, andelectrical requirements—as overall mirror or aperture size rises.Additional details of such “scaling” may best be found from themathematical analysis presented in following pages.

In particular as will be seen the mirror/magnet inertia typically riseswith the fourth power of their diameter—driving up the response time—butresponse also falls linearly with the mirror/magnet diameter, due to theradius on their disc where force or torque is normally applied. The neteffect of these two opposing tendencies is, roughly, an increase inresponse time with the diameter cubed—i. e., to for increase from 1 to 2cm diameter, nominally an eightfold increase in response time.

This approximate cubic function is found under the assumption that themagnetic flux per unit cross-sectional area is constant with increasingmagnet dimensions. Even this, however, is not the end of the matter asthe response is subject to further adjustment (usually downward) forincreased total flux in the magnet arising from such increasedthickness.

A person skilled in this field can estimate such additional adjustmentusing the equations presented later in this text. Since increasedthickness raises the available flux and therefore torque, theinteraction of these several considerations leads to existence of anoptimum mirror/magnet thickness—that cannot be simply read from thegraphs of FIGS. 11 and 40, but should be found by applying all themathematical constraints outlined in this document.

Other practical considerations arise, as for example the effect of airgaps in the magnetic circuit—at the inboard ends of the radial arms. Forbest response, to minimize these gaps the radial arms are brought asclose to the central magnet (and mirror) as possible.

The magnet/mirror itself may be circular, as indicated elsewhere in thisdocument. Instead, and preferably, that assembly may be elliptical sothat it presents a substantially circular profile when partly rotatedout of the plane of the overall structure—particularly when viewed alongthe normal to that structure. Such out-of-plane effects can be exploitedto maximize torque and thus minimize response time when the mirror is atangles that demand highest torque for good response time.

If the magnet/mirror assembly is circular, its adjacent air gaps arebest minimized by forming spherical surfaces at the inboard ends of theradial arms, to match the three-dimensional surface swept out by thosearm ends. If instead the assembly is elliptical as noted just above,then a more-sophisticated ellipsoidal surface at the inboard arm endsmakes a better match.

FSM FEEDBACK DESIGN—The feedback design of our preferred sensor has beendescribed as a position sensing detector (“PSD”). The mirror-positionfeedback device can be any type of device that provides the requiredresolution and dynamic response for the desired application. Some suchdevices are photodetector arrays, linear CCD arrays and the like.

MAGNETICS DESIGN—In modeling and designing the drive system of fastscanning mirrors (“FSM” or “MSM”) according to preferred embodiments ofour invention, two components come into play:

flux density (B), generated by a magnetic circuit; and

magnetic moment of a magnet

Combining these models together enables calculation of the torque on themirror. Then, given the torque and the inertia of the FSM, we can findthe acceleration and response times. Below we provide an example ofthese modeling and design procedures.

MAGNETIC-CIRCUIT MODEL OF FSM—The magnetic-circuit model is a physicallyaccurate, realistic representation of the apparatus that makes up ourpreferred FSM steering assembly. It includes a central, circularpermanent magnet 411 (FIG. 33), preferably sintered NdFeB.

Closely adjacent to that magnet are four coils 412 (412 a through 412d), each wound on a respective radial mu-metal core. In the prototypepreferred embodiment of our invention the cores 412 are each 6.35 mm indiameter, and separating each core from the central magnet is a 1.53-mmair gap 413.

Disposed circumferentially around the four prototype cores, with theircoils, is an octagonal mu-metal magnetic feedback path 416 with squarecross-section, 6.35 mm across and 6.35 mm tall. This feedback path 416(familiarly designated the “racetrack”) is in intimate contact with theoutboard ends of the four cores 412, and carries magnetic flux 414between the several core ends.

More specifically, magnetic flux from the outward end of arepresentative core 412 a flows to the racetrack 416, dividing itselfinto clockwise and counterclockwise paths 415 to the diametral core, i.e. to the aligned core 412 d at the far side of the magnet. The fluxthen proceeds through that latter core 412 d radially inward—across theassociated air gap—to the central magnet.

The length of one such return path 415 (i. e. either a clockwise or acounterclockwise path) from air gap to air gap, in our prototypepreferred embodiment, is 102 mm. In simplest principle the flux bypassesthe pair of intermediate cores 412 b, 412 c that are at right angles tothe two mutually diametral cores 412 a, 412 d just mentioned.

Critical to good operation of this apparatus is understanding (andwell-designed control) of the magnetic signal in the feedback paths415—ideally following the mathematical analyses set forth below. Ourmost-highly preferred embodiment controls the magnetic signal bypulse-width modulation (“PWM”) of the driving signal. Although we useanalog sensing of the magnetic flux in the feedback channel, we isdigitize the various signals to implement the system in a semiconductorprocessor—i. e., we prefer digital control of the PWM; however, apartfrom convenience of using such a processor, analog current control couldbe used instead.

The above magnetic circuit, in turn, for a helpful visualization of itsoperation can be modeled mathematically as an “electrical” circuit.Specifically, the electromagnet around the mu-metal core is modeled as avoltage source Ni (FIG. 34) with a resistor R_(c) in series. The airgaps are modeled as resistors R_(g); and the permanent magnet, as acurrent source φ_(r) with a resistor R_(m) in parallel.

The core reluctance (R_(c)) is given as:

$R_{c} = \frac{l_{c}}{\mu_{c}A_{c}}$

-   -   where:        -   l_(c)=length of core        -   A_(c)=cross-sectional area of core        -   μ_(c)=permeability of core (mu-metal)            The air-gap reluctance (R_(b) or R_(g) is written:

$R_{g} = \frac{g}{\mu_{0}A_{g}}$

-   -   where:        -   g=air-gap size        -   A_(c)=cross-sectional area of air gap        -   μ₀=permeability of free space (or air)            The permanent-magnet mutual reluctance (R_(m)) is:

$R_{m} = \frac{d_{m}}{\mu_{m}A_{m}}$

-   -   where:        -   d_(m)=diameter of magnet        -   A_(m)=cross-sectional area of magnet        -   μ_(m)=permeability of magnet,            assuming that the remanence flux density of the permanent            magnet in the direction of this circuit is zero.

(“Remanence” is commonly defined as “the magnetization left behind in amedium after an external magnetic field is removed . . . . It is denotedin equations as M_(r). In engineering applications it . . . isfrequently denoted as [residual flux density] B_(R) . . . ”Wikipedia.org, 2008)

The flux φ flowing through that magnet is then:

$\phi = {\frac{Ni}{R_{c} + {2R_{g}} + R_{m}}.}$To find the flux density in the air gap, we divide this flux by thecross-sectional area of the gap:

$B_{g} = {\frac{\phi}{A_{g}}.}$This variable is a linear function (FIG. 35) of current in the coils.

AIR-GAP GEOMETRY—Since the geometry and size of the air gaps have alarge impact on the performance of the magnetic circuit, we considerthem in further detail here. To find the cross-sectional area of the airgap, we start with the geometry of the gap.

At the outside of the central mirror and magnet 411, the adjacentarc-shaped inward end 412′ of each radial core or “arm” 412 subtends anangle θ (FIG. 36):

$\theta = {2{{\sin^{- 1}\left( \frac{w_{c}}{2r_{c}} \right)}.}}$

For our 10 mm FSM, this angle is θ=1.0155 rad. Then with the arc 412′length along the outside of the mirror given by:

$L = {\theta\left( \frac{d_{m}}{2} \right)}$—and, assuming that the angular motion of the FSM is independent ofmirror diameter, the air-gap area is:

$A_{g} = {{L \cdot g} = {{{\theta\left( \frac{d_{m}}{2} \right)}g} = {1.0155\left( \frac{d_{m}}{2} \right){g.}}}}$

MODEL PERMANENT MAGNET AS AIR-CORE COIL—The equation for is the fluxdensity generated by a coil 417 (FIG. 37) with an air core is:B _(z)=μ₀ NiNext, to model the permanent magnet 411 as an air-core coil we set theremanence flux density (B_(r)) of the magnet to B_(z) and solve for Ni.

${Ni} = {\frac{B_{r}}{\mu_{0}}\mspace{25mu}\left( {\text{amps/}\text{meter}} \right)}$(amps/meter)Now assuming a single loop 418 (N=1) at the radius of the magnet and awidth of the magnet equal to its length l,

$I = {\frac{B_{r}}{u_{o}}l\mspace{31mu}{\left( \text{amps} \right).}}$(amps).The magnetic moment of the magnet is:

$\overset{\rightarrow}{m} = {{\pi\left( \frac{d_{m}}{2} \right)}^{2}I{\hat{z}.}}$Torque acting on the magnet (FIG. 38) is:{right arrow over (T)}={right arrow over (m)}×{right arrow over (B)}_(g).

FSM PERFORMANCE ESTIMATES—To find the angular acceleration of themirror, we divide the torque by the combined inertia of the magnet andmirror. The inertia increases with the magnet/mirror thicknessquadratically; and the torque, linearly. The mirror inertia, however, isgreater than that of the magnet. These relationships lead to a peak(mentioned earlier), in the acceleration as a function of magnetthickness (FIG. 11)—since the torque increases faster than theincreasing inertia—but as the magnet thickness continues to grow, theinertia begins to overwhelm the torque.

The illustrated function is applicable to a mirror and magnet ofdiameter 10 mm. These acceleration figures can be used to determine theachievable response time (FIG. 40). This graph, too, applies to a 10 mmmirror and magnet, with no ring around the magnet.

ELECTROMAGNET PERFORMANCE EXPERIMENTS—The goal of this work was tocompare several diverse magnetic-feedback coil designs to one another.For most-meaningful comparisons, we wanted the geometries (in particularthe air-gap sizes) for the several coil types to be substantiallyidentical. This was accomplished using a test structure (or “jig”) 421(FIG. 41) quickly made on a so-called “3D printer”.

Such a “3D printer” device is a variant of a stereo-lithography machinethat extrudes layers of plastic. (Ours uses the type of plastic known as“ABS”, rather than curing resin.) It is commercially available as ModelSST-1200 from the company Stratasys, Inc.—sometimes instead calledDimension, Inc.—of Eden Prairie, Minn. People skilled in this field willunderstand that many other ways of preparing our test structure couldproduce wholly equivalent results.

With this structure, a Hall-effect sensor 424 (used to measure theflux-density strength) was locked in place relative to therepresentative test coils 412 a, 412 b—within a recess 422 formed in thejig 421. Three screws 423 held the octagonal magnetic circuit 416 or“racetrack” to the jig; and we used two-part epoxy adhesive 435 a, 435 b(FIG. 42) to held the coils 412 a, 412 d and sensor 424 to the jig 421.

The coils were driven with a current-limited power supply 433 (FIG. 42).The Hall-effect sensor 424 was driven by a separate power supply 432,and the sensor output monitored with an oscilloscope 431. The sensoritself was a Sentron 2SA-10 with magnetic sensitivity of 50 V=1 T.

In our simple experimental setup, a test module 410 e (FIGS. 41 through43) comprising the two-hundred-winding test coils and the octagonalracetrack 416, was supported on the jig 421; and the latter was in turnplaced on a tabletop 420. Two mating test-coil modules 410 c (FIG. 43,at left and right) of another design—the mu-metal core with asecond-generation (“GenII”) return, discussed below—were also similarlymounted and tested. Each controlled a respective one of the twomirror-rotation axes. Yet another tested module 410 d hadhundred-winding coils.

The values we were reading were extremely small (so small that systemdrift interfered with each measurement). Therefore we adopted thismeasurement procedure, changing the current by 0.5 A increments:

1) Turn the output off at the power supply

2) Set a current-limit value on the power supply

3) Set the scope acquisition mode to “average”

4) Set REF1 to “channel 1” to rebaseline the measurement

5) Turn the supply output back on

6) Record the power-supply current and voltage

7) Record the mean delta voltage from the oscilloscope

8) Return to step 1 and iterate for a new current setting.

We followed this procedure for five configurations (FIG. 43). The“air-core coils” were, as the title implies, coils with no solid core atall. The design entitled “Mu-metal core with no return” did have acore—made of so-called “mu metal”, which is known in the field—but thecore was not enabled to provide any “magnetic feedback path” 416 (FIG.33).

The third title, “Mu-metal core with FSM GenII return”, refers to oursecond-generation (“GenII”) prototype. For completeness here, we mentionthat the precursor first-generation “GenI” prototype (not shown) was themodule, mentioned elsewhere in this document, that used an older Drapermirror assembly. In addition, we fashioned two related units 410 d, 410e with a mu-metal core that also used a so-called “racetrack” return. Ofthese two, in one unit 410 d the number of coil turns was a hundred; andin the other unit 410 e, two hundred.

No measurable flux was found using the first design (the air-corecoils). Measured flux density can be considered as a function of current(FIG. 44) for the remaining four designs. The steeper the slope of thegraphed line, the greater the flux density supplied per unit current (i.e., higher gain). Our graph shows that while the GenII return has thehighest slope 441, unfortunately it saturates quickly. The racetrackdesign has lower initial slope 442, but (much more favorably) does notsaturate until the flux density is three times higher. By increasing thenumber of windings (to 200 turns from 100) in the racetrack design, wecan make the slope 443 approach the GenII slope 441, but again withoutsaturation until about 1 mT.

Since energy-conversion efficiency is also important, we monitored thepower-supply power as well, and plotted flux density against power (FIG.2). This plot shows that the racetrack design has about three timesgreater flux-carrying capacity than the Gen-II design.

AFOCAL BEAM STEERING—An important category of systems enhanced andfacilitated by preferred embodiments of our present invention is thecategory previously known as “AMBS” (afocal MEMS beam-steering) systems.As pointed out earlier in this document, however, the present inventionsweeps more broadly than MEMS (microelectromechanical systems).

Preferred embodiments of the present invention in general favor thejewel-bearing construction that—as noted above—has far better responsecharacteristics, including dynamic range. Also strongly preferred arethe other low-friction bearings discussed above.

The use of afocal optics to enlarge or reduce the field of regard,however, remains a particularly valuable feature of the preferredembodiments. Since those embodiments encompass both MEMS and nonMEMSsteering systems, the acronym “AMBS” is now best modified by droppingthe “M”—leaving simply “ABS” for “afocal beam steering”. The advantagesof this feature, already set forth in other patent documents of ourcoowned series, will be reprised now.

In preferred embodiments, the invention provides a low-cost sensorsystem 210 (FIG. 21) capable of detecting and locating activeillumination sources—or objects illuminated by such sources. In somepreferred embodiments (FIGS. 22 through 24), the sensor system of theinvention can also respond to the detected light source by returning alight beam 238 (FIG. 23) or even an object, and in some cases byinitiating a distance-determining or other investigation (Function 4,FIG. 24) of the source or objects associated with the source.

In particularly preferred embodiments initial detection of a radiationsource or illuminated object is qualified by filters that implementexpectations as to the characteristics of such sources or objects thatare of interest. For instance, when anticipated sources are infrared, orare in other particular spectral regions, spectral filters are placed atconvenient positions in the optical path—usually but not necessarilyassociated with the fold mirror 221, and in particular taking the formof bandpass optical reflection/transmission filters.

Because our novel fast scan mirror 215, 216 (FIG. 21) has a very highrange of angular deflection, we can now build an optical system thatfully covers the desired field of regard even though the system has noafocal element at all. That is, in purest principle the afocal element214 can simply be omitted. People skilled in this field will appreciate,however, that some practical considerations may limit application ofthis principle without appropriate compensations for the differentoperating conditions.

In a system according to our invention and with no afocal element,incoming radiation 213A (FIG. 21A) from a representative source 201Aproceeds directly to the steering mirror 215A, 216A. The mirror controlsystem 226A is calibrated appropriately for the different relationshipsbetween mirror angle and external pointing direction, in the absence ofan afocal element.

As mentioned elsewhere in this document, for various purposes the foldmirror 221 (FIG. 21), FIG. 221A (FIG. 21A) can be advantageouslyimplemented as a beam splitter, and incident-beam selectivity is simplyan additional one of such purposes. In such arrangements, if it ispreferable that certain spectral components of the beam not pass to theprimary directionality detector 224, a dichroic or other bandpass orbandblocking filter can be used, as an alternative to a fold mirror 221.The filter transmits these undesired components to a radiation sink orto an auxiliary is detection system 255, while reflecting the desiredradiation components to the detector—or conversely, depending onpreferred system configuration.

Such advance filtering is not limited to spectral characteristics. Byway of example, if anticipated sources are modulated temporally, thesignal 225 from the optoelectronic detector 224 is advantageouslyfiltered electronically 256 to exclude d. c. sources or sources havingno significant bandwidth activity above a specific thresholdfrequency—or, more restrictively, to pass only a. c. signals having aparticular specified modulation pattern or class of patterns.

Ideally the system detector 224 is a PSD, which has the ability toreport positional coordinates ΔX, ΔY (on the PSD's own surface 225) ofan impinging optical beam from a source 201 in a region without thenecessity of scanning the region. As noted elsewhere in this document,it is also necessary to determine the mirror position. From these dataand known characteristics of associated optics, as explained above,angular position θ_(X), θ_(X) of the source is readily calculated.

As mentioned earlier, however, a PSD is nonlinear and temperaturesensitive when measuring large off-axis coordinates ΔX, ΔY and thusangles θ_(X), θ_(Y). Such drawbacks are neutralized, in preferred formsof the present invention, by operating in a null-balance mode asdetailed below—so that the system relies on the PSD primarily only todetermine whether the source is off axis and, if so, then in whichdirection; and not for quantitative reporting of large off-axiscoordinates or their associated angles.

After the sensor system (including the arithmetic preprocessingmentioned earlier) has determined initial values for the incident anglesθ_(X), θ_(Y), the system very rapidly servocontrols itself to keepincident rays 213 at the center of the detector field. Most preferablysuch servocontrol 227 is implemented by one or moremicroelectromechanical (MEMS) mirrors or fast scanning mirrors (FSM) 215disposed inside the optical system 210, i. e. along the optical pathbetween the detector 224 and the collecting aperture 214, 245 (FIG. 25)of the system.

Such mirrors have extraordinarily low mass and inertia, andcorresponding extremely high response speed—thus obviating the problemof sluggish response in earlier systems. Additional observations as tothe new, larger MEMS or FSM mirrors appear later in this section.Placing the mirror or mirrors inside the system gains yet furtheradvantages of angular displacement speed, in the visible volume 211 ofspace outside the optical system, particularly if a lens 245 is placedat the aperture to optically magnify the angular displacement of themirrors.

This particular arrangement for servocontrol of the incoming light, tocenter the beam on the detector, is particularly advantageous when usinga PSD. Whereas that type of detector measures large off-axis anglessomewhat inaccurately, the system is easily made extremely accurate inmeasuring the angular correction 228 applied by the MEMS or FSM systemto bring the source to the central, null position.

Throughout most of this document, for purposes of simplicity thenotation “θ_(X), θ_(Y)” has been used to represent both the off-axisangle of the beam 213 (FIG. 21) and the mirror-orientation 216 signals228 relative to nominal or rest positions of the mirror or mirrors 215.As will be understood, these two different sets of angles 213, 228 arenot at all the same—but when the system has servocontrolled itself tonull the incident beam at the center of the detector 224, the two setsare related by straightforward transforms. Such transforms include themagnification factor introduced by the afocal package 214, as discussedat length elsewhere in this document, and also include the localcalibration of the mirror actuator-stem positions relative to aninternal standard, and also distortion in the afocal array 214 as wellas the final focusing optic 223, and so forth.

The PSD itself can effectively monitor a far larger angular region 211than it can image. This is a major advantage never fully exploited inconventional systems because of failure to use internal mirrors, or verysmall mirrors, and because of failure to servo the input source to areproducible centerpoint on the detector.

Nevertheless a still further major advantage is gained by rasterscanning 216 the PSD. The basic principle behind this is that the systemviews a small part of the field of regard at any instant in time, yetexpands its coverage by searching for incident rays, thereby coveringthe entire field of regard 211. As will be seen, practical field of viewusing the various forms of the invention can range, representatively,from 20° to 180°.

This combination of the intrinsic angular-dynamic-range advantage of thePSD with the multiplicative advantage of a raster scan yields anenormous bandwidth, or bit depth, in overall determination ofoptical-source angular location θ_(X), θ_(Y). Moreover, once a lightsource 201 is detected and the MEMS or FSM mirrors operated by aprocessor 226 to center the source in the detector field, advantageouslythe processor sends the MEMS or FSM mirrors further signals to continuesearching/scanning 216 in the general area of the detected rays—withoutlosing the benefit of having the source near the detector center, wheremoderate angular accuracy is available. (Alternatively the nativeangular range of the PSD can be used for this purpose without additionalmirror scanning.)

The optical system has been successfully servocontrolled to an incidentray when both coordinates ΔX and ΔY (FIG. 25) of the ray on thesensitive detector surface are zero as measured by the two-dimensional(“2-D”) detector assembly (or in the case of a 1-D detector, when ΔX orΔY is zero and the scan-mirror positions are noted). Once the system isservoing to the incident ray, as noted above it can function todetermine not only angular location of the incident ray but also itswavelength λ and coded temporal modulation f(t); or can direct similaror different light rays 235-238 (FIG. 23) opposite the incident rays 213along the same path, or laterally 243 with respect to that path—forcommunications, distance determination, optical enhancement or otherpurposes. In the case of light rays received from an adversary forguiding an object with destructive intent, an auxiliary laser 242 can bedirected 241 to emit a very bright beam 243 of identical wavelength λand temporal modulation f(t) onto a nearby (but progressively diverging)surface. This arrangement can closely mimic the original beam but in adifferent guiding location, and thereby draw off the object from theintended destination.

Various arrangements can be used to bring the auxiliary laser intooptical alignment. One such arrangement is a variable-position foldmirror 221, 221′ (FIGS. 22, 23 and 25); however, for simultaneousoperations as noted earlier such a mirror can be replaced by a beamsplitter, e. g. a polarized one for maximum radiation transfer, or byspectral-band-wise splitting devices such as dichroic filters.

The sensor system is ordinarily located on a host 510 (FIG. 24). Anappropriate host is readily selected to optimize use of the inventionfor particular applications. In preferred embodiments, the host can be avehicle including an automobile or truck, sea vessel, airplane,spacecraft, satellite or projectile, or even simply a human or animal ortheir paraphernalia. Hosts are not limited to these examples, but canbasically consist of any carrier—even stationary—capable of supportingand maintaining the sensor, and exposing it to various kinds of articlesor objects.

The sensor method or system specifications can vary and be optimized foruse in particular applications. One of ordinary skill in the art canselect preferred configurations of the system to suit a particularapplication. In preferred embodiments of the invention, the system canmonitor a field of regard at approximately 10 Hz frame rate—evidencingthe excellent sensitivity of the invention at high frequencies. Theinvention is capable, however, of monitoring in a range on the order of1 Hz to 1 kHz—or even 10 kHz, depending on size of articles of interest,and the detector field of view. Overall, the invention provides a highdegree of angular accuracy in determining the approach path of anincident ray.

Plural such sensor systems can be grouped and coordinated to provide upto 4π steradian coverage—i. e., for sensing in all directions at once.This kind of observation is appropriate for a host that is in the air orin outer space, and in some circumstances for a host that is waterborne.For a host on land, and for a water-surface-craft host in othercircumstances (particularly, no need to monitor below the water surface)2π steradian coverage ordinarily is entirely sufficient.

The sensor of the invention has the ability to monitor wavelengthsranging from ultraviolet (UV) to infrared (IR), particularly up to themidIR range.

Typically a MEMS mirror is limited in range to plus-or-minus ten tofifteen degrees about one or two orthogonal axes, i. e. through anoverall excursion 216 of roughly 20° (FIGS. 25) to 30° for each axis. Inour preferred embodiments, as noted earlier, a lens assembly 214 isadvantageously used to significantly increase this range optically.

Most preferred embodiments of the invention eliminate the use of largeexternal scan mirrors and macroscopic gimbals; as a result the inventionis more rugged, and yet actually less expensive and several orderslighter and more compact than conventional sensor systems. For example,the size of the system, depending on the application, can be on theorder of one millimeter, or less, to a few centimeters—rather than onthe order of one centimeter to tens of centimeters as described earlierfor conventional units.

Prior to introduction of the new, larger MEMS mirrors introduced in theBernstein and Taylor patent documents discussed above, dimensions of anoscillating scan mirror 215 could be, merely by way of example, in arange from a few tens of microns wide to several millimeters or more;such a mirror could be roughly square, or could have a high aspect ratiosuch as 25:1 or 50:1. Nominally and ideally, however, the aspect ratiowas approximately the square root of two, since the mirror surface—whenat the center of its range of excursions—is usually, or mostconveniently, inclined at 45° to both the incident and reflected beams.These observations as to aspect ratio hold true for the new generationof larger, magnetically driven mirrors as well.

Accordingly, while the older mirrors in their most preferable testedembodiments were e. g. silicon scan mirrors in the range of 1.5×2.1 mm(note that 1.5√{square root over (2)}≅2.1), the newer mirrors areadvantageously 5 mm and above, in the shorter dimension, and 5√{squareroot over (2)}≅7 mm and above in the longer dimension. The magneticallydriven mirrors are capable of construction up to (in the shorterdimension) preferably 10 mm, and more preferably 20 mm, and still morepreferably even 30 mm, 50 mm or greater. The corresponding longerdimensions are respectively fourteen, twenty-eight, forty-two andseventy-four millimeters or more.

Again, these dimensions are not at all limiting. The earlier MEMSmirrors, as used in our invention, typically rotated about their ownaxes with excursion in the range of ±1° to ±10°—or even ±15° aspreviously noted.

The newer units, having a capability for greater clearance above thebase as explained in coowned earlier documents, are also capable of muchgreater angular excursion such as ±30°. Although this added mobility isquite valuable in the environment of our invention, the greatestadvantages of the newer mirrors lie simply in their larger dimensions—asthese very greatly simplify major increases (well over an order ofmagnitude) in optical-energy throughput, leading to correspondingadvances in signal-to-noise ratio and diffraction limit.

Such improvement in diffraction limit pushes the imaging sharpness toexcellent new values exemplified elsewhere in this document (namely,very fine resolution and imaging sharpness), as is well understood fromclassical diffraction is analysis. Nevertheless these mirrors are alsocapable of assembly into a multimirror array having quite stunningoverall optical-transmission area.

Furthermore such an array in turn is amenable to the diffraction-controltechniques set forth in the related '015, '869 and '103 provisionalpatent documents mentioned earlier. These documents teach how to forcesuch a mirror array to operate at the diffraction limit of a singlemirror of like overall size.

As set forth in our related patent documents, one way to achieve suchenhancement is to use a wavefront sensor to directly measureirregularities in optical waves reflected from such an array—and then torefine the adjustments of all the mirrors in the array so that themirrors all have (1) a common angle and (2) negligible relative-phasedifference between adjacent mirrors in the array.

This second condition, negligible “relative” phase difference, allowsoverall pathlengths at the different mirrors to differ—but neverthelesseliminates phase degradation over the array by holding the differingadjacent pathlengths to very nearly 2π radians. Expressed inmathematical terminology, the pathlength differences at adjacent mirrorsare “modulo 2π”.

As also set forth in the related documents, a second way to achieve thesingle-mirror diffractive effect is to provide an auxiliary opticalpath—that traverses the main-path mirror set—and test the sharpness ofimaging with an auxiliary laser beam in that path. A preferred way ofaccomplishing this is to generate a point-spread function or “PSF” forthe auxiliary beam, and adjust the mirrors in the array in such a way asto optimize the PSF. The result is again the modulo 2π relationship foradjacent mirrors.

As a practical matter, this condition is met adequately when thedifferences are within ten to twenty percent of one wavelength of modulo2π. This and other details are explained in the provisional applicationsearlier mentioned.

The system mass can be made just one-tenth to one kilogram, which isalso generally several orders of magnitude lower than that of comparableknown devices. Angular resolution is readily placed in thesubmilliradian or even tens-of-microradians range, i. e. less than threeminutes of arc or even under one minute, versus the previously notedtens to hundreds of milliradians (two-thirds of a degree to tens ofdegrees) for sensors heretofore. Yet another major and remarkableadvantage is that the system can eventually use off-the-shelftechnology, requiring no expensive custom parts or instrumentation.

Initially, as noted in our copending precursor '595 application, themost highly preferred embodiments of the invention called for a custommirror array of at least 5×5 mirrors—and more preferably 10×10 and even30×30 mirrors—each individual mirror being at least 30 microns in theshorter dimension, up to at least 1.5 mm in that dimension and 2.1 mm inthe longer, and with an afocal lens assembly that follows custom opticalspecifications but is otherwise conventionally fabricated. Using largermirrors, 5 to 30 mm in the shorter dimension, it is possible to achievesubstantially equal overall area with a much smaller number ofmirrors—for example smaller in proportion to the square of the mirrorlinear dimension.

Since the linear dimension of the newer mirrors is typically greater bytwo to three orders of magnitude, the number of mirrors may be four tosix orders smaller for the same collecting power. With thisunderstanding, a previous mirror array of even 30×30 mirrors is equaledin area by less than one of the newer units. In short, one large mirrore. g. 311 or 312 (FIG. 31) potentially does the work of many previous,smaller mirrors.

Another approach to exploiting the potentially much greater size ofmagnetically driven mirrors, however, is to aim for a much largeroverall steering-mirror area. For example, an array of 10×10 mirrors ofthe newer type comes to 100 times the collecting power of a single newmirror, and this in turn is six to eight orders' greater area thanpreviously achievable. The result is enormously enhanced overallperformance, in terms of both signal-to-noise ratio and (assumingadequate provisions for diffraction control) imaging sharpness.

A representative array may for example comprise six such mirrors111-116, for example with two parallel, spaced-apart unitarily formedflexures 317 a, 318 a (FIG. 32), and resulting rotational axes 317, 318(FIG. 31). As will now be clear to people skilled in this field, analternative is an end-to-end two-mirror array 311-312 with likewiseparallel, spaced-apart rotational flexures 317 a, 318 a and axes 317,318; or a side-by-side two-mirror array 311, 314 with flexures alignedend-to-end to produce one common rotational axis 317—or a side-by-side,three-mirror array 314, 311, 316, likewise with a single common axis317.

Skilled people in this field will further see that many otherconfigurations are possible and usable, each with its to own combinationof advantages and tradeoffs. In the representative single mirrors 311, .. . 316, as well as the representative array 311-316, advantageouslyeach mirror e. g. 311 or 312 is provided with coils 311 c, 311 d—or 312,312 d—that may be energized substantially independently to createrespective magnetic fields, at the two opposite sides of thecorresponding axis 317 or 318.

These created fields in turn interact with other magnetic fields (e. g.fields of permanent magnets that may be in the base 319, or elsewhere asdescribed in the Bernstein-Taylor documents) to develop separatemagnetic forces 311 a, 311 b—or 312 a, 312 b—acting on each mirror atopposite sides of its rotational axis. In the case of oppositelydirected forces 312 a, 312 b it is readily appreciated that the forcesboth urge the mirror 312 into rotation in a common angular direction,which may produce (all other things being equal) maximum torque and thusmaximum angular-velocity response, for minimum electrical input power.Currently such a geometry and functionality appear ideal for, at least,a single-mirror system.

In precursor documents it was observed that then-favored componentdesigns were expected to “quickly become standard in the field, and veryshortly be available as commercial off-the-shelf units.” In view of thenew developments in MEMS-mirror or FSM technology, that observation mustnow be seen as unduly conservative—that is, the earlier designs havebecome not only standard but outpaced by the newer mirrorconfigurations.

Most of the advances described herein, in practicing our presentlight-detecting, -characterizing and -response invention (i. e. indetailed construction and use of the invention) will be straightforwardand clear, based upon the information in this document—consideredtogether with the teachings of the other patent documents mentioned inthe “RELATION BACK” and “RELATED DOCUMENTS” sections at the beginning ofthis document. One part of the invention, however, may bear specificelaboration here, and that is the use is of magnetically controlled MEMSor FSM mirrors to implement the fullest forms of diffraction control:

Piston Adjustment in a Magnetic MEMS Array

As made plain by coowned earlier patent documents, wholly incorporatedherein, the control of diffraction to optimize imaging sharpness—with asteering mirror array—requires simply matching of certainadjacent-mirror conditions. Specifically, although matching the angles(called “tip” and “tilt” angles) of adjacent and nearby mirrors 311, 312(FIG. 32) in the two rotational axes of the mirrors (only the “tilt”angle being shown) is critical for common pointing, and is also helpfulto providing a partial smoothing of optical wavefronts, another kind ofmatching appears essential to full diffraction control.

That kind of matching is along, roughly, the piston dimension P of thearray. By “piston” we mean the positioning of a mirror in the directionP normal to the common base (and rest plane) of the mirror array.

Depending on the particular steering angle which the array is commandedto provide, the ability of the array to produce an essentially coherentreflected wavefront requires coordinated fine adjustment in the tip,tilt and piston directions—all three. This coordination is required toensure that the modulo-2π condition is satisfied for the direction inwhich the reflected beam travels (or perhaps more accurately an averageof the directions of the incident and reflected beams).

The need for some adjustment in the piston direction is may berecognized from the distance Δ (FIG. 32) between the tips of theadjacent mirrors 317, 318. That distance may be conceptualized asmeasured along the true piston direction P (normal to the base and tothe mirror rest plane), or alternatively along a normal to the commonplane of the rotated mirrors.

Another preference is to measure the distance along theincident/reflected average direction mentioned above; and this is,roughly, the convention illustrated in FIG. 32. However measured, thedistance A is in general disruptive to the required in-phaserelationship of wavefront elements or “wavelets”—in the overallreflected beam—because that distance, in general, fails to be a multipleof one wavelength, or even close to such a multiple.

Without correction, therefore, the wavefront elements from thoseadjacent mirror tips must in general interfere with one anotherdestructively, at least in part. Given such interference the diffractionlimit for each mirror is determined, substantially, by the linear extent(in each principal direction) of that mirror considered individually.

Our object, by contrast, is instead to condition the diffraction limitbased upon the linear extent of the overall array. The well-knownrelationships of diffraction thereby provide a finer, sharper focus ofeach object imaged.

In purest principle, one way—which may perhaps be regarded as a“trivial” way, or alternatively as an extreme and very impracticalway—to avoid or minimize partially destructive interference is tomechanically align the adjacent edges of the adjacent mirrors. In otherwords, one (or both) of the mirrors in theory can be moved to perfectlyaligned positions 311, 312′ (the latter being shown in the is brokenline).

Since the mirrors are at the same angle, such alignment (if possible)actually would cause the mirrors all to form one single reflectivesurface, and constructive interference would be guaranteed. What makesthis approach impractical and extreme is that, in general, extremelylarge piston excursions would be required.

Fortunately it is not necessary to attempt such an approach. It isnecessary only to bring the adjacent mirror edges into, approximately,optical-phase alignment—not mechanical alignment. As long as thewavefront elements propagating from the two adjacent edges aresubstantially in phase, desired diffraction relationships obtain andimaging sharpness is very greatly enhanced.

Within a typical mechanical interedge distance Δ, for mirrors orientedat representative angles, typically there are many hundreds or thousandsof optical wavelengths. Therefore the number of opportunities to find asubstantially in-phase relation is typically an extremely large number.

By “substantially” in phase we mean wavefront elements in phase withinabout ten percent of one wavelength—part of a highly preferableembodiment of our invention. Even just holding the phase error undertwenty percent is usually or often adequate for a significantenhancement of image quality, and offers a somewhat less-preferableembodiment of the invention.

In summary, the piston-direction adjustment of e. g. mirror 312 need notat all appear as in FIG. 32, where that mirror has been moved byessentially the entire distance A. A very tiny fraction of that distancegenerally suffices, and as already noted the optical-phase alignmentneed not be exact.

In any event, it will be clear that, at least in general, wavefrontcoherence demands adjustment in the piston direction. The term “piston”here is particularly apt, in that most MEMS mirrors in the earliergeneration of nonmagnetically controlled devices actually had a kind ofmechanical piston component, i. e. an extensile rod element, thatphysically protruded or extended in, generally, the dimension directlytoward and away from the base (and rest plane) of the array, along thenormal.

Accordingly “piston adjustment” referred literally to mechanicallymaneuvering that piston (or its connecting rod) for a net excursioninward and outward from the base. We say “net” because some MEMS-mirrorconfigurations depend, in those earlier devices, upon operation of thepiston or rod to achieve the tip and tilt adjustments too.

The interdependence of tip, tilt and piston controls persists in thecurrent generation of MEMS devices, but in these devices the mechanismsof these adjustments are different. We shall now detail those newmechanisms.

It has been explained earlier that each mirror can have not just one buta pair of electromagnetic control coils 111 c, 311 d (FIG. 31)—or 312 c,312 d—for forcing the mirror into rotation; and that passage ofelectrical currents through these coils in appropriate directions andmagnitudes can—in effect—twist the mirror about the correspondingrotational axis. Here by the term “twist” we do not mean to suggest thatthe mirror is necessarily deformed in a twisting mode, but rather onlythat the whole mirror is bodily rotated by forces e. g. 312 a, 312 bapplied separately at its two half-panels or lobes that are at oppositesides of the rotational axis 318.

Those forces 312 a, 312 b, when considered from the point of view oflinear directions, are oppositely directed—but when considered from thepoint of view of torque about a central axis of rotation they are,generally speaking, operating in a common direction and additive. Inother words, even though the two electromagnetic forces—assuming thatthey are oppositely directed in the linear sense—oppose each other inthe linear sense and tend to cancel (to the extent that they are equal),they instead augment each other and tend to supplement each other in therotational sense.

In this way the electromagnetic forces 312 a, 312 b adjust the mirror toa particular rotational balance point, a particular angle, that pointsthe reflected or incoming beam (or both) in a particular direction—andalso, as suggested just above, may be used to set the diffractiveperformance for a smoother wavefront.

Now, to the extent that those two oppositely directed forces are notequal, they do not cancel linearly; instead there is some residual ofthe larger one (e. g. 312 b) of the forces that adds in the linear senseto the smaller one (e. g. 312 a). In this case there is a net force inthe (negative or positive) piston direction, so that the entire mirroris bodily drawn in toward the array base 319 or (as illustrated) thrustoutward away from that base.

This net force and the resulting motion, and positioning of the mirror,toward or away from the base is in fact “piston adjustment”. If such netforce is opposed by other forces, for example restoring force due tospringiness of the rotational flexures used to enable rotation of themirror, adjustment of the net electromagnetic driving force resultsdirectly in a specific “piston” position, which can be calibrated.

If springiness in the flexures is not sufficient or suitable to producethis desirable result, then other sources of restoring force (evenincluding counteracting forces produced electromagnetically, if desired)are readily provided—as will now be amply clear to people skilled inthis field. Also it will now be clear to such people that the forcesproduced by electrical current through the respective coils, in theirinteraction with permanent magnets as discussed in the above-mentionedpatent documents, are readily adjusted in such a way as to achieve anydesired or needed combination of tip, tilt and piston settings.

Another way to represent the rotational and piston forces is to show thetwo forces 311 a, 311 b acting on the two ends of a single mirror 311 asboth pointing in a common linear direction (FIG. 31)—particularly,outward from the base. This conceptualization perhaps more naturallyexplains the piston suspension of the mirror outward from the base. Withthis basic arrangement, a controllable incremental difference betweenthe two forces 311 b, 311 a serves to rotate or “twist” the mirror 311about its rotational axis 317—once again producing a desired combinationof piston, tip and tilt adjustments.

Setting array mirrors e. g. 311, 312 for approximate optical in-phaserelationships as described above does require some basis for determiningwhen two adjacent wavefront elements are, or are not, in phase—or,alternatively, determining when the overall system is as well tuned asit can be. Our previously identified related patent documents describehow to do this.

A first way, also shown very generally in this present document, is toprovide an auxiliary optical system 331-339, which in effect emulatesthe behavior of the main functional light paths through the overallsystem—but does so in a way that enables direct measurement and thusoptimization of imaging quality. Thus a laser 331 directs an auxiliarybeam 332 to a first beam-splitter 333, which in effect acts as a foldmirror, forwarding the laser beam 334 toward the mirror array 311-316exactly parallel to the main optical path 321-322, 323-324.

After redirection (with, presumably, some wavefront distortion) by thearray, the reflected beam 335 reaches a second splitter 336, and from itpasses through a beam-conserving optical element 338—fully discussed inthe co-owned '103 application—to an imaging detector 339. Focal elements(not shown) bring the laser beam 332, 334, 335, 337 to a focus on thatdetector 339.

The quality of that focus is developed as a so-called “point spreadfunction” or “PSF”. Thus the degree of perfection of the PSF as receivedat the imaging detector 339 serves as a figure of merit for the variousmirror-disposition adjustments.

Various algorithms enable perturbation of the mirror adjustments tooptimize the PSF. This step is preferably performed for each of a greatnumber of steering angles of the two-axis mirror array, and the resultsstored in memory so that ideal adjustments can be very quickly andprecisely summoned back for any particular steering-angle combination.

A second way to determine the phase quality of the mirror adjustments isto insert a wavefront sensor 351 directly into the main optical path321-322, 323-324—as set forth in the coowned '015 patent document. Herethe above-discussed auxiliary path 331-339 is omitted, and instead thewavefront sensor (typically including a sensor array) determines phaseimperfections of the overall wavefront from the mirror.

In this case as well, the mirror settings can be perturbed according toany of various protocols, to optimize each part of the wavefrontseparately; or, alternatively, to optimize the overall quality in a moreholistic fashion—as, for example, using a neural network. As with thePSF technique, such measurement is best performed as a function ofsteering angles, so that optimized tip, tilt and piston settings can beautomatically recalled as soon as a pair of steering angles is invoked.

The use of such magnetically developed forces 311 a, 311 b or 312 a, 312b—or both—together with mirror-setting optimization apparatus 331-339 or351, serves to enable a steering-mirror array 311-316 to achievediffraction-limited performance corresponding to the dimensions of theentire array—and at the same time to very rapidly and nimbly canvass abroad field of regard, through a wide field of view. This isaccomplished by the combination of large angular deflections of thesteering-mirror array itself, further magnified by an afocal optic 314.

The optic directs light collected from external objects along an opticalpath 321, 323 to the mirror array, and thence along the further path322, 324 to a focusing lens and then a detector 324. Various detectortypes are appropriate, depending upon the particular operationspreferred—as set forth in other sections of this document.

Several other optical subsystems are advantageously incorporated intothe system of our invention, individually or in combination. They arediscussed in other parts of this document or the related documentsintroduced earlier.

Any such other subsystem, or any combination or subcombination of them,may be placed at an appropriate position e. g. 351 along the mainoptical path. Such a subsystem may be placed in advance of the focusinglens 323 and detector 324, or may be included instead of the lens anddetector, or if preferred may have access to the optical path inparallel with the lens and detector, via a beam-splitter (not shown).

For instance such a subsystem or combination or subcombination 351 mayinclude, without limitation:

-   (1) a wavefront sensor as mentioned earlier;-   (2) a spectral-analysis module;-   (3) a ranging laser for projecting a ranging beam to an object at    unknown distance outside the optical system, and a ranging-laser    receiving module, distinct from the detector and focusing lens, for    receiving and analyzing the ranging beam after reflection from such    an object;-   (4) a communication-beam transmission module for transmitting a    first modulated communication beam toward such an object, and a    communication-beam reception module, distinct from the detector and    focusing lens, for receiving and interpreting a second modulated    communication beam received from the object or from a region that    includes the object;-   (5) a powerful laser for projecting a beam to impair function or    structural integrity of an external object;-   (6) a laser for dazzling or confusing either a human operator or    optical apparatus associated with such article, or both; or-   (7) an auxiliary optical system that includes an imaging reception    module, distinct from the detector and focusing lens, for receiving    and interpreting an image beam received from such an object or from    a region of such volume that includes such an object.    Preferably the afocal optic 314, and the mirror 311 (or array    311-316), are shared by the detector 324 with its focal lens 323 and    by the subsystem, combination, or subcombination 351.

From the foregoing discussion it will be appreciated that our presentinvention provides a new method of operating an optical system. Themethod includes providing 171 (FIG. 20) an array of magneticallycontrolled dual-axis rotatable MEMS steering mirrors. Each mirror isconsidered to have separate electrical coils disposed at opposite sidesof a rotational axis.

The method also includes providing 172 at least one other magnet whosemagnetic field interacts with magnetic fields created by the coils. Theinteraction produces magnetically generated forces.

The method further includes the step of directing 173 electricalcurrents to the separate coils of each mirror, to produce at least twocomponents of magnetically generated forces, including:

-   -   a pair of variable forces directed in opposite linear        directions, applying variable torque to the respective rotatable        mirror, and    -   an additional variable net force tending to thrust the        respective mirror outward from (or draw it inward toward) a rest        plane of the array, causing variable piston movement of the        respective mirror.

An additional step is adjusting 174 the at least two components offorces so that the steering mirrors direct a light beam in a desiredsubstantially common direction, and so that light-beam wavefrontportions from adjacent steering mirrors are substantially in phase.(Foregoing discussions explain how this is done, including ways ofmonitoring and responding to the wavefront or the imaging qualitydirectly.) The result is to achieve a diffraction limit conditionedsubstantially by the entire array dimension rather than an individualmirror dimension.

As the preceding sections of this document make clear, this method hasapplication in various functions, some of which receive light beams foranalysis and others transmit light beams for imaging or other kinds ofinvestigation. In either case, preferably another step of the method ispassing 175 the light beam through an afocal optic to changemagnification of steering-mirror rotations. Still another step isdetermining 176 characteristics of the light beam when received or as(or after being) transmitted.

Often the several steps of this method are performed by respectivelydifferent people or institutions, or in respectively different locationsor at respectively different times. The core steps of the method—stepsperformed once the full system is in place, but before the ultimateutilization steps (passing 175 and determining 176) may perhaps best beregarded as the directing step 173 and adjusting step 174.

Resolution, FOV, Dynamic Range, and Other Parameters

Remarkably, even though the present invention (including forms of theinvention set forth in precursor applications) by virtue of itsexcellent diffraction characteristics achieves far finer resolution (forexample from twenty-five to fifty microradians) than earlier sensors, atthe same time it nevertheless also provides much broader effective fieldof regard (for example ninety degrees or roughly 1.6 radians). Thesedual advantages can be stated together in terms of an extremely higheffective dynamic range (for example from roughly 32,000:1 to 64,000:1).

In the FOR-reducing embodiments of our invention, as explained elsewherein this document, finer pointing precision is possible. In physicalprototypes to-date, we have demonstrated precision of one milliradian.Ultimately, however, precision as fine as ten to one hundredmicroradians is a realistic goal. We believe that we will achieve thelower figure.

The invention can redirect a new beam 243 (FIG. 23) of light (usuallygenerated locally—i. e. on the same platform) laterally for guidance ofany objects away from the host. The invention can also providedetermination of wavelength λ and frequency-modulation information f(t)in the received beam, so that those characteristics of the received rayscan be mimicked 241 in the new beam—which is relayed to anotherlocation, either for communications purposes or to lead an approachingobject to a different destination. Alternatively the new beam can bedirected back along the same path 238 as received rays 213, to theextent that the field of regard of the optical system (or of the systemtogether with other such optical systems being operated in parallel) isbroad enough to provide appropriate directions for the new beam. Thesecapabilities are entirely beyond those of the prior art.

Preferred embodiments of the method of the invention, corresponding tothe apparatus discussed above, include the steps or functions of:

-   -   detection and angular location of a light source (FIG. 21),    -   determining characteristics of the received radiation (FIG. 22),        and    -   response (FIG. 23).

The first of these functions preferably includes these constituentsteps:

STEP 1—Incident rays 213 from a light source 201 illuminate the system,on its host platform, at a relative angle θ_(X), θ_(Y).

STEP 2—What we ordinary call a “magnifying” afocal lens assembly 214does in fact magnify a collimated or nominally collimated incident orexiting ray angle, θ_(X), θ_(Y) (i. e. outside the optical system) bythe ratio of the two focal lengths designed into the assembly, 1:3 inthis example. This change results, however, in much smaller off-axisangles of θ_(X)/3, θ_(Y)/3 inside the optical system 210—i. e. at thescan mirror or mirrors 215. This arrangement is optimal to effectively,or virtually, bring the incident rays within the native scan range ofthe MEMS scan system.

The lens assembly 214 is described as “afocal” because it is not used tofocus the incoming rays directly onto the detector 224; rather theprimary lens 245 forms (inside the lens assembly) only a virtual image244, which the secondary lens 246 then recollimates—but only if theincoming beam 213 a, 213 is itself at least approximately collimated—toproduce substantially parallel rays in the beam approaching the detectorassembly 222.

STEP 3—The MEMS scan mirror continuously raster-scans the field ofregard. When the MEMS scan mirror intercepts laser energy at thecorresponding original angles θ_(X), θ_(Y) (and reduced angles θ_(X)/3,θ_(Y)/3), the detector detects the energy and in turn transmits thesignal to the control processor. The relative position reported at thatsame instant by the MEMS scan mirror assembly, and thereforecorresponding to θ_(X), θ_(Y) is recorded by the control processor 226.To enable this result, a conventional two-axis angle sensor (not shown)that measures shaft angle of the MEMS mirror has been precalibrated toprovide the corresponding field of regard angle (θ_(X), θ_(Y)) relativeto the optical axis.

STEP 4—The 2-D detector is fitted with a reimaging lens that focuses theincident beam at its conjugate location on the detector, relative to thesystem axis, provided that (1) the MEMS scan mirror is at an appropriateangle to direct the beam into the detector field of view, and (2) theincoming beam, within the envelope of extreme captured rays 213, 213 a(FIG. 25), is collimated or very nearly so. This arrangement tends tosomewhat diffuse the image of relatively nearby sources on the detector,and thus limit the response to light from relatively remote sources.,

The detector is thus aided in essentially disregarding illumination fromnearby sources, which for purposes of preferred embodiments of thepresent invention are most-typically considered irrelevant. (As will beunderstood, contrary assumptions can be implemented instead, if desired,in other—generally conventional—optical trains.) Such exclusion ofillumination that is not of interest, however, is generally secondary inrelation to other selective features in the system—e. g. spectralfiltering 221, 255, and a. c. signal filtering 256 or other arrangementsfor enhancing sensitivity to anticipated known modulation patterns.

The position-sensing detector next comes into play, sensing not onlypresence of the illumination but also the displacements ΔX, ΔY of itsfocal point (conjugate location) from the optical axis—and generatingcorresponding ΔX, ΔY signals for transmission to the control processor.

STEP 5—Mirror-bias commands Δθ_(X), Δθ_(Y), proportional to the ΔX, ΔYvalues, are generated by the control processor and sent to the MEMSscan-mirror assembly. These signals drive the conjugate locationapproximately to the optical axis; and as that location approaches theaxis the error signals ΔX, ΔY become progressively more linear andstable, by virtue of the inherent behavior of the PSD 224, so that theeventual determination of incident-beam location is extremely precise,accurate, and stable. At each instant the source angles outside theoptical system are related to the coordinates on the PSD surface by thefinal-stage focal length, i. e. each angle Δθ_(X) or Δθ_(Y) equals thecorresponding ΔX or ΔY coordinate divided by the 2-D detectorimaging-optic focal length f_(D) (FIG. 25)—subject to the angle-scalingeffect of the afocal assembly 214, discussed at “step 2” above.

STEP 6—The Δθ_(X), Δθ_(Y) incident-ray relative position as thenmeasured by the MEMS scan-mirror local angle sensors are made available,for later functions, as an accurate line-of-sight location of theincident ray relative to the system axis.

The second function of the system basically includes determining thewavelength and any accompanying temporal or spectral modulation of theincident ray or signal. Continuing the above sequence:

Step 7—A fold mirror 221 (FIG. 22) rotates to direct the incident beam213 to a spectrometer or photodiode 231. The fold mirror is basically asimple, motorized mirror that redirects light; but in other preferredembodiments this mirror can be replaced by a MEMS mirror or, as notedearlier, a beam splitter. One or more splitters, in tandem asappropriate, are particularly advantageous to permit simultaneousoperations of different types, e. g. detection, spectral analysis,imaging, distance probing, or active response—and combinations of these.

Step 8—A spectrometer 231 determines the incident ray wavelength. Thedetector in the spectrometer may acquire any temporal or spectralintensity or wavelength or temporal modulation to be detected and sent232 to the control processor. Portions of this task may be assigned tothe PSD 224, filter 256 (FIG. 21) and processor 226 for data acquisitionduring earlier steps 5 and 6.

The third system function is most typically an optical response that cantake any of several forms. One form (FIG. 23), which makes use ofdirectional information collected in the first function, is generationand projection of a very bright beam of radiation opposite the incidentray, to temporarily dazzle or confuse an operator or aiming apparatus atthe source. Again continuing from the first-function sequence:

STEP 7—The fold mirror 221 (FIG. 23) rotates from its earlier positions221′ to align a powerful laser 234 along the optical axis, and therebyalong the known path to the source.

STEP 8—The laser transmits a temporarily blinding beam 235-238 in adirection opposite the incident rays 213, but back along the same path,in response to a command 233 from the control processor 226.

A fourth function uses the information collected in the second functionto generate and project a precisely wavelength-matched andtemporal-modulation-matched beam to a nearby location, preferably onethat progressively moves away from the host position, to draw any guidedobject away from the host. Friendly as well as hostile guided rendezvouscan be facilitated in this way. This fourth function includes issuanceof a processor command 241 (FIG. 23)—with necessary data λ, f(t)—to theauxiliary light is source, e. g. tunable modulated laser 242. Atsubstantially the same time the determined information is advantageouslytransmitted (preferably as interpreted, encoded data) to a remotestation to document, e. g. for subsequent refined avoidance, what hasoccurred.

As will be understood, if the application at hand calls for directing abeam into the originally searched input volume 211, rather than alocation laterally offset from that volume, then instead of theauxiliary laser 242 it is possible to use the previously mentioned laser234—i. e., the one that can be aligned with the main optical paththrough the lens assembly 214. This option is particularly practical inthe case of a plural-sensor-system apparatus configured to scan 2π or 4πsteradians as previously discussed. In such applications essentially alllocations are within the scanned range of at least some one of thecomponent sensor systems.

A complex of other possible responses, and alternative applications ofthe information gathered in the first two functions, is within the scopeof the invention (FIG. 24). One such response is initiation of adistance probe operation to collect additional information about anysuch object that may be associated with the beam, or about facilities atthe source, or both. Several of the references cited at the beginning ofthis document provide very extensive information aboutdistance-determining capabilities and design. Other ranging methods maybe substituted as desired. This form of the invention can also be usedfor any of various other applications, such as transmission of modulatedoptical signals for free-space laser communications.

For each of the various applications additional components may be added,such as additional processing capability for further processing data, anannunciator for alerting an operator or connecting to an alarm formonitoring the system, or robotics to perform additional functions inresponse to detection.

Particularly preferred applications, as shown, include use of the systemin a vehicle or other host for detection of objects, or use of thesystem as a guide for a laser communications telescope—for which thesystem “communicates” angular, wavelength, frequency-modulation (orother temporal modulation) or other information between two telescopes.Also included is use of the system for continuous observation purposessuch as recognition and location of emergency distress signals e. g. abeacon, or flares, or identification of approaching vehicles.

Furthermore the system can detect such light signals in outer space oreven through large bodies of water. Thus objects can be identified andlocated regardless of whether they are floating in space, under the seaor on land. Other beneficial uses will appear from the drawing; however,it is to be understood that FIG. 4 is not intended to be exhaustive; i.e., not all functions of the invention described and discussed in thisdocument appear in that drawing.

Because of the versatility of the system and its many functions, it hasa wide range of applications spanning industries as diverse astelecommunications, optics, automotive, marine, aerospace, continuingobservation, and search and rescue.

In a particularly preferred embodiment of the invention as set forth inthe copending precursor application, the sensor system utilizes atwo-axis scan mirror (FIG. 225) of dimensions 1.5×2.1 mm, withmechanical scan angle of plus-or-minus 10° to 15°—for a total excursionof 20° to 30°—about both axes. These various values, however, andrelated values, are preferably supplanted by those appropriate to thenewer MEMS or FSM mirrors as detailed above. A two-axis scan mirror isnot a requirement; a single-axis scan mirror with one-dimensionaldetector can be substituted. Using a two-axis scan mirror with a 2-Ddetector, however, allows greater flexibility in detecting throughout avolume or detecting in more than one dimension.

A ±10° or ±15° sweep 216, i. e. 20° or 30° full-excursion, of the MEMSor FSM mirror or mirrors 215 is doubled—by the effect of reflection—toproduce a 40° or 60° deflection of the beam at that point. The MEMS/FSMsystem, in turn, is behind a lens assembly whose focal-length ratio(typically 1:3) triples that 40° or 60° deflection to provide,typically, a 120° to 180° overall field of regard. The two-axis MEMSscan mirror, operating at approximately four milliradians forapproximately the magnification (again, typically three) times 2λ/d,repeatedly sweeps the full 120°×120° volume at more than 10 Hz. Thisthen is the frame rate for a complete scan of that field of regard.

If a collimated or nominally collimated incident ray is directed towardthe host within this overall field of view, the ray is projected—throughits reimaging lens—onto the detector when the MEMS or FSM two-axisscanning mirror is at the corresponding angular position. Thescan-mirror control system then drives the scan mirror to maintain theincident ray on the detector, ideally a position-sensing photodiodedetector as described earlier—and preferably at its center.

This detector provides positional closed-loop feedback to the scanmirror, driving the focal point to minimize the ΔX and ΔY coordinates.In other words the beam is driven to the native origin on thephotosensitive surface of the diode.

When in that condition, the angular positions of the mirror provide thecorresponding azimuth and elevation angles Δθ_(X), Δθ_(Y) of theincident rays—based on the corresponding error coordinates ΔX, ΔY at thedetector surface, and the corresponding known relative mirror angles asexplained earlier. Limiting uncertainty of the input collimatedlaser-beam angle is the limiting resolution of the 2-D detector dividedby the reimaging lens focal length f_(D).

In addition to illuminating the PSD, the system advantageously includesa multiposition relay mirror (or fold mirror etc.) to alternativelydirect the incident beam to other detectors such as a spectrometer usedto determine incident-ray wavelength—or a beam-splitter to do soconcurrently. If preferred, quad cells, focal plane arrays, or linearrays such as a charge-coupled device (CCD) or other light sensitivearrays can be used instead. Ideally each individual detector of an arraycan be provided with its own individual microlens. Nevertheless thepreviously mentioned quantization effect remains a concern, and arraydetectors are generally slower than PSDs, particularly when taking intoaccount the necessary algorithmic procedures for readout andinterpretation of optical signals.

The same multiposition mirror can also serve to route output rays, froman onboard laser or other bright lamp, back along the original opticalpath toward the source of the initially detected incident beam—to blindthe source operator, or locate the source facility, or communicate withit, all as set forth earlier.

In practice of many of the preferred embodiments of the invention—butparticularly for situations in which the system cannot lock on to anactive source, usually because no active optical source is present ornone is being concurrently detected and tracked—it is especially helpfulto provide a vibration-sensing subsystem 257 (FIGS. 21 and 22) adjacentto the scan mirror or mirrors, and a correctional-data path 258 for flowof vibration information from the outputs of these sensors to the mainprocessor. (Although included in FIG. 21, such provisions most typicallyare in order only when no positional detection is available, e. g. as inFIG. 22 with the detector 24 out of service, or absent. Vibrationsensing 257, 258 and input filtering 255, 256 are omitted from FIGS. 23and 25 only to avoid further clutter in those drawings.) This sensingmodule 257 with its correction path 258 enables a spectrometer, or animaging system or distance-determining system, that is part of theinvention embodiments to form a stable, high-resolution 2-D or 3-D imagedespite vibration in the host platform.

Most typically the vibration sensor includes a gyroscope or set ofaccelerometers, separated by known lever arms. These devices provideenough information—most typically with respect to five degrees offreedom—to enable the system to incorporate compensating maneuvers ofits moving mirrors, canceling out the effects of such vibration. Thesedevices should be augmented by a GPS sensor for geodetic coordinates.

Sensing elements 257 positioned along the plane of a supporting base ofthe moving mirror or mirror assembly 215 can for example include threelinked accelerometers sensitive to motion normal to that plane; and twoothers, to motion in that plane—ordinarily but not necessarily parallelto orthogonal edges of the base. Such vibration-sensing devices ineffect define instantaneous characteristics of any host-platformvibration. Such sensing subsystems in themselves are well known andconventional. The data they produce must flow to the processor 226 andbe interpreted promptly enough to enable effective feedback into thecontrol circuits of the moving mirror or mirrors, to achievecancellation within desired imaging accuracy of the overall system.

Vibration sensing, like other functions involving detection of relativeposition as between the MEMS/FSM mirrors and the base or platform—whenusing the newer, magnetically driven mirrors—ideally may be performedthrough use of magnetic pickups, e. g. auxiliary coils built into theindividual MEMS mirrors. This sensing strategy is particularly favorablefor the same reasons that the magnetic mirror drive itself isadvantageous, namely that action and sensing at a greater distance ismore practical with magnetism than with mechanical, electrostatic orpiezoelectric phenomena.

For most purposes of the present invention, as previously mentioned,raster scans are advantageously performed using a spiraling pattern 259(FIG. 26). With moving mirrors, executing such a pattern is mosttypically far more energy-efficient and fast than tracing amore-conventional rectangular-envelope serpentine pattern. For optimumspeed and efficiency the sequence reverses direction at each end—i. e.,outward in one scan, inward in the next, and so forth. As in any rasteroperation, the number and pitch of the spiral revolutions should beselected with care to obtain good resolution without significant gaps inthe image.

DETAILS OF THE ROVING FOVEAL SENSOR APPLICATION—In certain preferredembodiments of the invention the objective system is capable of pointingover a full-hemispherical field of regard (FOR). Concurrently preferredembodiments of this invention can deliver geolocation information, basedon platform position and attitude derived from an inertial-navigationsystem (“INS”).

The most highly preferred embodiment of our invention utilizes an afocaloptical front end with a hemispherical FOR. In this approachservoed-beam steering—according to the coowned patent documents notedabove—provides the active pointing capability within the FOR.

The term “foveal” refers to the high-resolution portion of the humanretina, particularly the portion at the center of the retina. A relatedphrase is “foveated camera”.

According to certain preferred embodiments of our invention, a “rovingfoveal” camera—consisting of two or more detector planes providing botha wide field of view (“WFOV”) and a steerable narrow field of view(“NFOV”)—can be designed using an appropriately configured opticalsystem, an optical beam splitter and a fast steering mirror. (Forpurposes of this document, “WFOV” is essentially synonymous with “fieldof regard” [“WFOR”], or wide field of regard [“WFOR”].)

Preferred roving-foveal embodiments of our invention are addressedprimarily in separate coowned patent documents, covering three basicconfigurations. The present document takes up only one of thoseconfigurations, particularly one that appears to be most directly ormost naturally associated with the mirror innovations presented in thisdocument.

In this configuration, first, WFOV and NFOV imagers 91, 97 (FIG. 18) canshare a common optical aperture 85, 86 using a beam splitter 88 (e. g. a50/50 splitter). This feature allows the optical radiation to traversedifferent pathways 89, 92-94-95 respectively corresponding to the twoimagers 91, 97.

Second, this class encompasses two different configurations, but againwe take up only one of those here: optical data for the WFOV imager 91are split out before the fast steering mirror 111, making that WFOVmovable in relation to the NFOV. (In purest principle, instead the NFOVoptical data could be split out first; however, such a variant could beawkward to design for the desired size relationships as between the twoimages.)

It is very specifically this movable-field capability that enables theNFOV high-resolution imager to “roam” or “rove” within, or in relationto, the WFOV image space. This mode of operation (as well as, therefore,this optical configuration) is now the most highly preferred embodiment.It gives an operator or system designer maximum flexibility to juxtaposethe plural scene views however the operator or designer may prefer, foroptimum information content.

In all or most cases the detector array can be sensitive at any opticalwavelength, and can be multispectral—and therefore limited only by theavailability of specific detector arrays (now and in the future).Additionally, the optical front end may if desired comprise an afocallens, most typically but not necessarily wide-angle.

As in the coowned prior patent documents mentioned above, the afocalfront end if present can be used to obtain large FOV angles unachievablewith the mirror alone (i. e. for steering mirrors that have an angularlimit).

Advancing imaging technology with electronically addressable pointing(without a large external gimbal box such as used in older conventionalsystems) provides the basis for such lightweight, low-power, andreliable imaging performance. According to our invention the objectivesystem can point over a hemispherical field-of-regard (FOR) whiledelivering geolocation/targeting information based—as mentionedearlier—on INS knowledge of platform position and pointing. Such asystem can provide enhanced low-cost imaging and pointing capability.

Because preferred embodiments of our invention can integrate feedbackfrom an onboard INS system, image stabilization is also possible. Thisability typically has not been available on small UAVs heretofore, sinceconventional stabilized imaging systems usually require scanning systemsthat exceed weight limitations for these craft. These new stabilizationand pointing capabilities in preferred low-weight, low-power embodimentsof our invention present a new set of possibilities for a great range ofusers—including but not limited to (as one extreme example) air-trafficcontrollers in a tower or a regional control facility, and (at anopposite extreme) soldiers on a battlefield.

Furthermore the simple provision of image stabilization facilitatesanalysis by ground operators and mitigates mental processing demands forinterpreting imagery. Finally, integration of the pointing system withthe INS enables object-locating capability (geolocation) within oursmall platform. Accuracy of geolocation is based on the accuracy ofplatform orientation (INS accuracy), and of platform position (INS andfor some applications an altimeter), and pointing accuracy.

Our invention contemplates sensor pointing accuracy on the order of 0.1mrad. For moderate INS performance of 7 mrad attitude precision and 5 mpositional precision (e. g. so-called “spherical error probable” or“SEP”), this implies approximately 8.5 m pointing precision (e. g.“circular error probable” or “CEP”) at 1 km ranges, in a planeperpendicular to the line of sight. This type of pointing location maybe used in a variety of ways, for example:

-   -   object coordinates may be handed off to another platform or        facility to perform any one of a wide range of application        objectives;    -   in applications requiring an immediate rapid response (e. g. in        collision avoidance or military contexts) a course correction or        a direct call for fire may occur; or    -   information may be simply cached for application planning        purposes to be performed at a later date.        For any new-capability innovation, new applications are often        developed by the user community. As systems in accordance with        our invention become smaller and more affordable, they become        more numerous in many use environments such as mentioned in this        document—and others as well. As a consequence, understanding        ways in which to best utilize them in a collective and networked        manner will also be recognized by people in these fields. This        concept offers an alternative to a large, centralized,        high-value-asset approach—with the possibility of equal or even        greater operational capability.

Preferred embodiments of our invention are compatible with a variety ofalternative applications, in addition to those already introduced. Oneexample includes replacing the afocal hemispherical optics with apanoramic annular lens, for stationary installations.

Further Technical Details

The technical objectives of preferred embodiments are best derived fromsystem objectives—e. g., a low-cost, lightweight (under 2 kg or 5pounds), low-power, persistent visible and infrared imaging and pointingcapability for UAVs and various other platforms. Our inventioncontemplates these system characteristics:

(1) fast optronic pointability,

(2) ±90-degree field of regard (FOR),

(3) in roving-foveal applications, a wide FOV (or FOR) for broad-regionimaging with continued monitoring, and a narrow FOV for pointing at andfollowing relatively small objects or regions,

(4) image registration/stabilization and object geolocation, and

(5) passive visible and IR capability, and active capability asdescribed in this document.

System Design

A key system-level objective, in implementing preferred embodiments ofour invention, is to develop a detailed system design and a performancemodel exhibiting integrated performance that satisfies theabove-mentioned descriptions. To avoid iterative efforts we recommendthat, based on the system design, elements necessary for a prototypedemonstration be developed. Specific goals of such a demonstrationideally include ability to perform not only pointing functions but alsopointing stabilization using INS inputs to support geolocation, asstated earlier. Although individual subsystem specific performancemeasures (e. g., wavefront error [“WFE”], response time, modulationtransfer function [“MTF”], stability, etc.) naturally should bedetermined individually, the overall system performance measurementsnoted here are best made in an operational context.

We advise against performing integrated system-design work without firstdemonstrating the key technology components individually. Thus werecommend starting any design effort with work on only theservoed-steering subsystem.

A much lower “risk”—but yet a major task and major design challenge—isdesign of the optical layout for the system. What is needed is an afocalwide-FOR design that produces images of sufficient quality to beoperationally useful. Within this design-tradeoff space is considerationof the spectral region or regions of interest and, for visible-spectrumsystems, the desire to color-correct the optical design. Since spectralbandwidth and color correction can entail major financial stakes, suchdecisions should be based firmly on overall system goals.

An exemplary color-corrected optical design (FIG. 3) has a ±10° scanangle for the two-axis servoed-mirror—and exhibits some of thechallenges faced in wide-FOV imaging. First, the effective entrancepupil of the system is compressed—which is a function of the angularmagnification considered together with the clear aperture of the servoedmirror. In this example, derived from our somewhat-earlier work, theeffective entrance pupil is on the order of 1 cm.

Another issue is related to the number of optical elements. Acolor-corrected system has a relatively large number of opticalelements, and this adversely impacts both size and weight objectives.This problem can be alleviated by going to longer wavelengths, orimposing single-wavelength operation on the system—but such mitigationscarry their own undesired limitations.

A final challenge exemplified by such an optical layout is the issue ofadequate image quality. In this example the MTF of the systemcorresponds to roughly fifty line pairs per millimeter, which is usuallythe minimum acceptable resolution for a detector array with pixels of 10μm pitch.

In the aggregate, such design constraints can be managed in only alimited number of ways. In the case of the effective entrance pupil,three factors can be traded-off in relation to the radiometricrequirements: total FOR, servoed scan angle, and mirror size.Simplification of the design by specifying single-wavelength operationis also a reasonable consideration—provided, naturally, that qualitycolor imaging is not required. Meeting image-quality requirements will,most typically, be a result of experienced optical-design efforts ratherthan rigorous scientific methods; however, our design goal of fifty linepairs per millimeter should be reasonably easy to meet, considering theperformance of the design under discussion.

Detailed designs should be explored to provide requirements flow-down tothe active optical-element development effort. Key parameters ofinterest are the diameter of each active optical element (or, forpolygonally or irregularly shaped components, a representative ormaximum transverse dimension).

As suggested above, we recommend that after design the system be modeledto verify that the design meets all objective and inferred requirements,such as radiometric properties and platform vibration. Althoughtypically a prototype system is intended as a laboratory demonstrationunit (e. g. breadboard unit) only, and not expected to be integratedonto a platform, ideally the design will show traceability to achievingtarget geolocation capabilities. Such “traceability” is bestdemonstrated and quantified through analysis in which pointing feedbackof the MEMS-mirror or FSM position is combined with the platformattitude, global position and knowledge of the local terrain to providean estimate of an object's coordinates.

The primary role of the servoed-beam-steering subsystem is to providepointing stabilization using INS inputs, to support image-based trackingfunctions. To that end the main objectives of the servoed-beam-steeringtask are to demonstrate a compact, low-power beam-steering package thatcan provide appropriate angular deflection and pointing accuracy forsystem requirements. It is further advisable to demonstrate closed-loopcontrol of the servoed-device, to provide active inertial stabilizationof the imaging system under realistic platform dynamics.

The previously mentioned CatsEye™ laser threat-warning program and ourpresent development activities have led to current preference for anoctagonal or ideally circular, electromagnetically driven mirrorassembly with an aperture of at least 1 cm (FIGS. 1, 2A, 4 and 5,) orpreferably 2 to 5 cm and even greater. The mirror may be amicromechanical type, e. g. MEMS with torsional flexures as illustrated,or preferably a fast scanning mirror (FSM) with ceramic or otherrefractory bearings. (As mentioned earlier, our previous work introducedthe extremely advantageous use of jewel bearings for steering mirrors;we have now noted that much the same benefits can be obtained with otherrefractory materials, replacing jewels. That advance is discussed indetail elsewhere in this document.)

The approach currently pursued has demonstrated mechanical deflectionrange (mentioned earlier) of ±22 degrees, utilizing a PSDmirror-position sensor for active positional feedback. This designprovides the necessary accuracy and dynamic range for imagestabilization and pointing, in the e. g., proposed system. A similarunit with a 1 cm square aperture (FIG. 2B) has successfully demonstratedtwo-axis, large-angle beam deflection. Mechanical improvements—inaddition to the square aperture—optimize spring stiffness.

Our early research and development work particularly explored the use ofHall-effect sensors to provide positional feedback for themirror-steering control system. Although Hall sensors initially appearedto show great promise, after extensive effort we have concluded thatother sensor types are far more suitable. The Hall-effect devices weinvestigated had only four-bit resolution, not at all adequate; andfurthermore were subject to very objectionable crosstalk—on account oftheir sensitivity to signals in the mirror-drive coils.

We have been much more satisfied with use of position-sensing detectors(PSDs) to monitor mirror position. This sort of sensing strategy can bestraightforwardly implemented in a system using our refractory-bearingmirror mount—especially since in that case we have access to the backside of the steering mirror. A separate mirror-monitoring light beam isdeflected by the back of the mirror, depending on the mirror angle, andreaches a PSD dedicated to monitoring mirror position. (In principlesuch a system can use the front of the mirror if preferred, but such anarrangement is sometimes partly incompatible with the best use of thefront of the mirror for deflecting the imaging or pointing beam.) Themirror-position monitoring path typically has its own radiation source,and the system reads PSD output signals directly to determine the actualposition on the PSD that is struck by the beam.

The PSD under discussion here is not the same detector used to measureobject direction or system-pointing direction, and in this(mirror-position measuring) situation it is permissible to measure beamposition off-axis relative to the detector—even though these units areprogressively nonlinear in position as an incident beam spot issuccessively farther from center. Linearity is not critical in suchmodules, as the calibrating relationship between mirror angle and PSDspot position can be rather fully characterized. Consequentlynull-balance operation, though very precise and accurate, is notrequired to calibrate the mirror angles very adequately.

In regard to the above discussion of aperture size, our mention of a 1cm aperture is intended as representative rather than as a maximum size.As explained in some of the coowned prior patent documents introducedearlier, we regard mirror sizes on the order of 2 cm, 3 cm, and more aspractical and desirable.

Since preparation of that document, however, we have considered designof mirrors that are 5 cm across, and even larger. We believe that, byexploiting the steering-mirror (and steering-deflector) designprinciples presented in this document, our invention has broken throughthe size limitations previously recognized as the state of the art.

Now, in view of that breakthrough, natural barriers to making steeringdeflectors of nearly arbitrary size seem very attenuated. We say thisconsidering, in particular, the possibility of using arrays of mirrors,rather than individual mirrors, as steering devices. As noted earlier,our investigations indicate that the overall size of steering arrays canbe readily increased well beyond five centimeters—to ten and evenfifteen centimeters—although in practice such deflectors nowadays mayhave little application, since five-centimeter units appear to servevery well.

On the other hand, we believe that in due course, devices according toour invention will prove very useful in a number of special situationssuch as rapid pointing toward relatively dim external articles, inpassive optical systems—and rapid steering of very bright responsebeams. For the time being, preferred embodiments of our invention haveparticular utility in a marketplace niche of very generally 5 mm to 5 or10 cm.

We believe that, when the large-mirror design techniques are combinedwith array techniques, the potential result is steering arrays on theorder of 15 cm or more, and diffraction-limit properties equivalent tothose of individual mirrors having such dimensions. Mirrors of suchsizes, however, are not limited to the torsional-flexure monosiliconconstructions introduced in that earlier document.

To the contrary, the larger mirrors are implemented even moreeffectively—much more—in the jewel-bearing embodiment of another of ourabove-introduced patent documents. We find that the jewel-bearing andother refractory-bearing embodiment is far superior in control-responsebandwidth, angular-setting stability when the mirror is not continuouslyheld in position by the control system, and related dynamiccharacteristics.

In addition, it appears that the larger mirrors as designed and used inour jewel- and other-refractory-bearing mounts can be significantly moreplanar in operation than the torsional-flexure units. As a result, alarger fraction of a radiation beam steered from these devices isactually pointed in the nominal direction and reaches the nominalposition.

Further, as detailed in our previous patent document dealing with thejewel- and other-refractory-system, performance of that system is muchbetter than the MEMS and other monosilicon options, in regard toresponse bandwidth, mirror planarity, directional controllability,linearity of directional adjustments. and stability of mirror directionwhen the control signals are removed.

The foregoing disclosures are intended as exemplary, not to limit thespecific configurations or operations of is our invention.

In certain of the appended apparatus claims, in reciting elements of theinvention in the bodies of the claims, the term “such” is used as adefinite article—i. e. instead of the word “the” or “said”—but only incross-references back to elements of the environment or context of theclaimed invention that first appear in the claim preambles. The purposeof this convention is to most-clearly point out those environmental orcontextual features, so that they are not mistaken for or confused withcomponents of the invention itself.

1. An optical system dynamically determining associated angulardirection throughout a specified range of angular directions, of anexternal article in a volume outside the system; said optical systemcomprising: a radiation source; an optical detector; an entranceaperture; an afocal element, associated with the aperture, enlarging thefield of regard of such external article and such volume as seen by thesource and detector; disposed along an optical path between (1)selectively, the source or detector and (2) the entrance aperture: atleast one mirror rotatable about plural axes and causing the source anddetector to address varying portions of such volume outside the opticalsystem; and each mirror of the at least one mirror having dimensions ina range exceeding five millimeters; wherein, due to said enlarging ofthe field of regard together with rotation of the at least one mirror,and substantially without changing magnitude of said enlarging, suchexternal article receives radiation from the source and returns saidradiation to the detector throughout the specified range; and whereinthe aperture, afocal element, and at least one mirror together form acommon optical path for said radiation from the source and to thedetector.
 2. The optical system of claim 1, wherein: said range ofmirror dimensions does not exceed fifteen centimeters.
 3. The opticalsystem of claim 1, wherein: the mirror dimensions are in a rangeexceeding one centimeter.
 4. The optical system of claim 1, wherein: themirror dimensions are in a range exceeding five centimeters.
 5. Theoptical system of claim 1, wherein: the mirror dimensions are in a rangeexceeding ten centimeters.
 6. The optical system of claim 1, wherein:the at least one mirror comprises a two-axis mirror in a gimbal-likemount having jewel, ceramic or other refractory bearings.
 7. The opticalsystem of claim 1: wherein the at least one mirror comprises a two-axismirror in a gimbal-like mount having jewel, ceramic or other refractorybearings; further comprising means for monitoring mirror position todevelop positional or rotational feedback information used in rotatingthe at least one mirror.
 8. The optical system of claim 7, wherein themonitoring means comprise: an auxiliary optical system that directs anauxiliary radiation beam to the back of the mirror and responds to theauxiliary beam after return from the back of the mirror, to determinerotational angle of the mirror.
 9. The optical system of claim 1,wherein: the at least one mirror comprises a two-axis mirror supportedin a gimbal-like mount having air bearings or magnetic bearings.
 10. Theoptical system of claim 1, wherein: the optical detector is aposition-sensing detector (PSD) that detects radiation returned fromsuch external article and tracks such article by inducing rotation ofthe at least one mirror to maintain such article centered on thedetector.
 11. The optical system of claim 10, wherein: the mirrordimensions are large enough to sharpen the diffraction limitsufficiently for beam divergence on the order of forty to fiftymicroradians or finer, at visible wavelengths.
 12. The optical system ofclaim 11, wherein: maximum transverse dimensions of the mirror are onthe order of three centimeters.
 13. The optical system of claim 10,wherein: the at least one mirror comprises an array of mirrors havingoverall array dimensions large enough to sharpen the diffraction limitsufficiently for beam divergence on the order of twenty-five to thirtymicroradians at visible wavelengths.
 14. The optical system of claim 13,wherein: maximum transverse dimensions of the mirror array are on theorder of five centimeters.
 15. The optical system of claim 1: whereinthe at least one mirror comprises a mirror array addressing such varyingexternal portions of such volume outside the optical system; and furthercomprising: a detector that detects radiation from such external articleand tracks such article by inducing rotation of the reflector tomaintain such article centered on the detector; and means for enablingdirect measurement and optimization of imaging quality; said enablingmeans comprising: an auxiliary optical system emulating the behavior oflight paths, to or from such varying external portions, at the array;said auxiliary optical system comprising a laser directing an auxiliarybeam to a first beam-splitter that forwards a portion of the beam energytoward the mirror array, parallel to said light paths to or from suchvarying external portions, and from the array to focus on an imagingdetector; and means for developing focal quality at the imaging detectoras a point spread function and using the point spread function as afigure of merit for adjustments of the mirror.
 16. The optical system ofclaim 15, wherein the developing- and using means comprise: means forperturbing mirror adjustments to optimize the point spread function, forplural steering angles of the array, and storing the optimized results;and means for retrieving the mirror adjustments for each optimized pointspread function, to reset the adjustments for any desired steering-anglecombination.
 17. An optical system dynamically determining associatedangular direction throughout a specified range of angular directions, ofat least one external article in a volume outside the system; saidoptical system comprising: a radiation source; an optical detector; anentrance aperture; an afocal element, associated with the aperture,enlarging or reducing the field of regard of such external article andsuch volume as seen by the source and detector; disposed along anoptical path between (1) selectively, the source or detector and (2) theentrance aperture: at least one mirror, rotatable about plural axes, andcausing the source and detector to address varying portions of suchvolume outside the optical system; and wherein each said at least onemirror comprises a two-axis mirror in a steerable mount having jewel,ceramic or other refractory bearings; wherein, when enlarging the fieldof regard, due to said enlarging together with rotation of said at leastone mirror, substantially without changing magnitude of said enlarging,such external article receives radiation from the source and returnssaid radiation to the detector throughout the specified range; andwherein the aperture, afocal element, and each said at least one mirrortogether form a respective common optical path for said radiation fromthe source and to the detector.
 18. The optical system of claim 17,comprising: means for controlling each said at least one mirror inrotation about the plural axes, to detect and track such externalarticles.
 19. The optical system of claim 18, wherein the controllingmeans comprise: a magnet fixed to or integral with the mirror; and meansfor applying variable magnetic fields to interact with the magnet andmagnetically develop torque that rotates the mirror.
 20. The opticalsystem of claim 19, wherein the controlling means further comprise:means for monitoring the mirror position to develop positional orrotational feedback information used in rotating the mirror.
 21. Theoptical system of claim 20, wherein the monitoring means comprise: anauxiliary optical system that directs an auxiliary radiation beam to theback of the mirror and responds to the auxiliary beam after return fromthe back of the mirror, to determine rotational angle of the mirror. 22.The optical system of claim 21, wherein the auxiliary optical systemcomprises: a position-sensing detector (PSD) that determinesdisplacement of the returned beam at the back of the mirror.
 23. Theoptical system of claim 21, wherein the auxiliary optical systemcomprises: an interferometer that counts fringes to determine positionof the mirror directly.
 24. The optical system of claim 17, comprising:means for controlling each said at least one mirror in rotation aboutthe plural axes, to follow a raster pattern for imaging at leastportions of such external volume.
 25. The mirror of claim 24, wherein:the raster pattern is a spiral pattern.
 26. The mirror of claim 24,wherein: the raster pattern is a spiral pattern that reverses direction,as between outward and inward spiraling, for alternate passes throughthe pattern.
 27. The optical system of claim 17, further comprising: abeam-splitter tapping a wide-field-of-regard, high-resolution image outof the optical system to a first imaging detector, at a point beforeincoming radiation in the system reaches the at least one mirror; asecond imaging detector receiving a narrow-field-of-view,high-resolution image from the at least one mirror; means forinterpreting resulting electronic signals from both imaging detectors,to display two nested portions of such volume on a single common visualmonitor; and means for controlling the mirror to position saidnarrow-field-of-view, high-resolution image selectively within saidwide-field-of-regard, high-resolution image on the single monitor.
 28. Amethod of operating an optical system that has a dual-axis MEMS steeringarray with individual mirrors having: transverse dimensions exceedingone centimeter, and separate magnetic-field-inducing coils at oppositesides of each rotation axis, and another magnet to interact withmagnetic fields induced by said coils; said method comprising the stepsof: directing electrical currents to the separate coils of each mirror,to produce for each mirror at least two components of force, including:a pair of variable forces directed in opposite linear directions,applying variable torque to the mirror, and an additional variable netforce thrusting the mirror outward from, or drawing the mirror inwardtoward, a rest plane or backing of the array as variable pistonmovement; passing a light beam through an afocal optic to changemagnification of the steering-array rotations; and determiningcharacteristics of the light beam when received or transmitted.
 29. Themethod of claim 28, further comprising a preliminary step of: selectinga maximum transverse array dimension of approximately three centimetersor greater; wherein the resolution is on the order of forty to fiftymicroradians, or finer.
 30. The method of claim 29, wherein thepreliminary step comprises: selecting the maximum transverse arraydimension as approximately five centimeters or greater.
 31. The methodof claim 29, wherein the preliminary step comprises: selecting themaximum transverse array dimension as approximately ten centimeters orgreater.
 32. The method of claim 29, wherein the preliminary stepcomprises: selecting the maximum transverse array dimension as notexceeding fifteen centimeters.
 33. The method of claim 28, furthercomprising the steps of adjusting the two force components so that: themirrors direct a light beam in a desired substantially common direction;and light-beam wavefront portions from adjacent mirrors are generally inphase, to achieve diffraction limit conditioned by the array dimensionrather than by the individual mirror dimensions.
 34. The method of claim33, wherein: said generally-in-phase light-beam wavefront portions arein phase within roughly ten to twenty percent of one wavelength.
 35. Amethod of operating an optical system that has a laser source fortransmitting a radiation beam to an external object, and has a dual-axisreflective steering device with overall transverse dimensions exceedingone centimeter, rotatable on jewel, ceramic or other refractorybearings; said method comprising the steps of: first, utilizing thedual-axis steering device to receive, and measure an incident angle of,an incident ray from the external object; second, utilizing thedual-axis steering device to direct such a radiation beam from the lasersource toward the external object in response to the received andmeasured incident ray.
 36. The method of claim 35, wherein the secondutilizing step comprises: directing such a radiation beam to disruptfunctioning or impair structural integrity of the external object. 37.The method of claim 35, wherein the first utilizing step comprises:operating the mirror at a peak of acceleration as a function of mirrorthickness.
 38. The method of claim 35, wherein the first utilizing stepcomprises: preparing the mirror and magnet with a diameter of roughlyone centimeter and thickness of roughly two or three millimeters. 39.The method of claim 35, wherein the first utilizing step comprises:preparing the mirror with diameter and thickness scaled from diameter ofroughly one centimeter and thickness of roughly two or threemillimeters, generally: in inverse proportion to fourth power ofmirror-and-magnet diameter, to account for inertia; and in linearproportion to mirror-and-magnet diameter, to account for location ofapplication of most driving force; for approximate net inverseproportion to the cube of the diameter, subject to further adjustmentfor increased flux in the magnet arising from such increased thickness.40. The method of claim 35, wherein the first utilizing step comprises:operating the mirror at a minimum of response time as a function ofmirror thickness.
 41. An optical system dynamically determiningassociated angular direction throughout a specified range of angulardirections, of an external article in a volume outside the system; saidoptical system comprising: a radiation source; an optical detector; anentrance aperture; an afocal element, associated with the aperture,enlarging or reducing the field of regard of such external article andsuch volume as seen by the source and detector; disposed along anoptical path between (1) selectively, the source or detector and (2) theentrance aperture: at least one mirror rotatable about plural axes andcausing the source and detector to address varying portions of suchvolume outside the optical system; and each mirror of the at least onemirror having dimensions in a range exceeding five millimeters; wherein,when enlarging the field of regard together with rotation of the atleast one mirror, and substantially without changing magnitude of saidenlarging, such external article receives radiation from the source andreturns said radiation to the detector throughout the specified range;wherein, when reducing the field of regard, pointing and steeringprecision is made finer by the ratio of reduction of the afocal element;and wherein the aperture, afocal element, and at least one mirrortogether form a common optical path for said radiation from the sourceand to the detector.
 42. The optical system of claim 41, furthercomprising: means for operating the system in a laser application forcommunications.
 43. An optical system dynamically determining associatedangular direction throughout a specified range of angular directionsthat defines a field of regard of the system, of an external article ina volume outside the system; said optical system comprising: a radiationsource; an optical detector; an entrance aperture; an afocal element,associated with the aperture, reducing the field of regard of suchexternal article and such volume as seen by the source and detector;disposed along an optical path between (1) selectively, the source ordetector and (2) the entrance aperture: at least one mirror rotatableabout plural axes and causing the source and detector to address varyingportions of such volume outside the optical system; and each mirror ofthe at least one mirror having dimensions in a range not exceeding fivemillimeters; wherein, due to said reducing of the field of regardtogether with rotation of the at least one mirror, such external articlereceives radiation from the source and returns said radiation to thedetector throughout the specified range; and wherein the aperture and atleast one mirror together form a common optical path for the radiationfrom the source and to the detector.